Python feature: config file to present Arbor's support on mpi, gpu and version (#708)

Fixes #700

Usage in python as: 

```
>>>import arbor
>>>arbor.config()
{'mpi': True, 'mpi4py': True, 'gpu': False, 'version': '0.1.1-dev'}
>>>d = arbor.config()
>>>arbor.print_config(d)
Arbor's configuration:
     mpi    :       True
     mpi4py :       True
     gpu    :      False
     version:  0.1.1-dev
```

python/CMakeLists.txt

@@ -16,6 +16,7 @@ add_subdirectory(pybind11)
 
 # The Python library. MODULE will make a Python-exclusive model.
 add_library(pyarb MODULE
+    config.cpp
     context.cpp
     exception.cpp
     mpi.cpp

python/config.cpp

+#include <pybind11/pybind11.h>
+
+#include "config.hpp"
+
+namespace pyarb {
+
+void register_config(pybind11::module &m) {
+
+    m.def("config", &config, "Get Arbor's configuration.")
+     .def("print_config", [](const pybind11::dict& d){return print_config(d);}, "Print Arbor's configuration.");
+}
+} // namespace pyarb

python/config.hpp

+#pragma once
+
+#include <arbor/version.hpp>
+
+#include <sstream>
+#include <iomanip>
+#include <ios>
+#include <pybind11/pybind11.h>
+#include <pybind11/stl.h>
+
+namespace pyarb {
+
+// Create and return a dictionary
+pybind11::dict config() {
+    pybind11::dict dict;
+    #ifdef ARB_MPI_ENABLED
+        dict[pybind11::str("mpi")]     = pybind11::bool_(true);
+    #else
+        dict[pybind11::str("mpi")]     = pybind11::bool_(false);
+    #endif
+    #ifdef ARB_WITH_MPI4PY
+        dict[pybind11::str("mpi4py")]  = pybind11::bool_(true);
+    #else
+        dict[pybind11::str("mpi4py")]  = pybind11::bool_(false);
+    #endif
+    #ifdef ARB_WITH_GPU
+        dict[pybind11::str("gpu")]     = pybind11::bool_(true);
+    #else
+        dict[pybind11::str("gpu")]     = pybind11::bool_(false);
+    #endif
+        dict[pybind11::str("version")] = pybind11::str(ARB_VERSION);
+    return dict;
+}
+
+void print_config(const pybind11::dict &d) {
+    std::stringstream s;
+    s << "Arbor's configuration:" << std::endl;
+
+    for (auto x: d) {
+        s << "     "
+          << std::left << std::setw(7) << x.first << ": "
+          << std::right << std::setw(10) << x.second << "\n";
+    }
+
+    pybind11::print(s.str());
+}
+} // namespace pyarb

python/pyarb.cpp

@@ -6,18 +6,19 @@
 // types and functions to be exposed to Python.
 namespace pyarb {
 
+void register_config(pybind11::module& m);
 void register_contexts(pybind11::module& m);
 void register_queryenvvars(pybind11::module& m);
 #ifdef ARB_MPI_ENABLED
 void register_mpi(pybind11::module& m);
 #endif
-
 }
 
 PYBIND11_MODULE(arbor, m) {
     m.doc() = "arbor: Python bindings for Arbor.";
     m.attr("__version__") = ARB_VERSION;
 
+    pyarb::register_config(m);
     pyarb::register_contexts(m);
     pyarb::register_queryenvvars(m);
     #ifdef ARB_MPI_ENABLED

python/strings.cpp

@@ -7,6 +7,18 @@
 
 namespace pyarb {
 
+std::string context_string(const arb::context& c) {
+    std::stringstream s;
+    const bool gpu = arb::has_gpu(c);
+    const bool mpi = arb::has_mpi(c);
+    s << "<context: threads " << arb::num_threads(c)
+      << ", gpu " << (gpu? "yes": "None") 
+      << ", distributed " << (mpi? "MPI": "Local")
+      << " ranks " << arb::num_ranks(c)
+      << ">";
+    return s.str();
+}
+
 std::string proc_allocation_string(const arb::proc_allocation& a) {
     std::stringstream s;
     s << "<hardware resource allocation: threads " << a.num_threads << ", gpu ";
@@ -20,15 +32,4 @@ std::string proc_allocation_string(const arb::proc_allocation& a) {
     return s.str();
 }
 
-std::string context_string(const arb::context& c) {
-    std::stringstream s;
-    const bool gpu = arb::has_gpu(c);
-    const bool mpi = arb::has_mpi(c);
-    s << "<context: threads " << arb::num_threads(c)
-      << ", gpu " << (gpu? "yes": "None")
-      << ", distributed " << (mpi? "MPI": "Local")
-      << " ranks " << arb::num_ranks(c)
-      << ">";
-    return s.str();
-}
 } // namespace pyarb