* Move label and s-expr parsing code and unit tests from Python to arbor C++ library.
* Update `locset` and `region` constructors that take string arguments to parse strings as s-expressions or "quoted" labels.
* Modify the input stream modifier used to parse asc files to use a lookup table for substitutions, renamed it transmogrifier.                                                             
* Replace `hopefully` type implemented in python headers with an `arb::util::expected`.
* Add `ARBDEV_COLOR` CMake option that forces gcc and clang to always output color output.
* Allow arbitrary strings in labels in region and locset expressions.
* Add `parse_region_expression` and `parse_locset_expression` functions alongside the existing `parse_label_expression` function for use when a region or locset is expected. These calls will promote a quoted string `"label"` to `(region "label")` or 
 `(locset "label")` respectively.
* Ad...

* Replace usages of `util::any` with `std::any`.
* Modify `util::any_cast` so that it forwards `std::any` arguments to `std::any_cast`; move into own header.
* Prevent `std::unique_any` from implicitly encapsulating `std::any` objects.
* Replace `test_any.cpp` unit tests with `test_any_cast.cpp` unit tests.
* Replace some usages of `std::is_same<X, Y>::value` with `std::is_same_v<X, Y>` as part of general C++17ification.
* Remove `util/variant.hpp`, `test_variant.cpp`.
* Replace occurrences of `util::variant` with `std::variant`.
* Replace `util::variant::get<N>` with `std::get<N>`.
* Replace `util::get_if<N>` with `std::get_if<N>`, passing a pointer to variant instead of a ref or value.
* Add some standard #includes where they were required in the source, but only included transitively.
* Some minor comment tidying and fix-ups.
* Update C++ API docs.

Fixes #1144.

Fix a Python example that was out of date.

* Remove spherical root/soma from morphology representations and logic.
* Replace `sample_tree` with `segment_tree`, which represents sample points in pairs and allows the representation of detached branches and cable segments.
* Remove sample point properties and predicates.
* Remove `cv_policy_flag::single_root_cv` as somata with a single attached dendrite will have a representation in the morphology of a single branch.
* Remove `arb::math::{volume/area}_sphere`, which are no longer being used.
* Remove spherical root special cases from `embed_pwlin` and `place_pwlin` objects.
* Add textual representation of `mnpos` in morphology exception text.
* Rename `sample_tree_from_swc` to `segement_tree_from_swc` and remove any special casing for one point somata.
* Remove `msample` and `mbranch` types.
* Update unit tests and python wrapper to reflect the changes.

A big update that wraps over 90% of the public C++ API, with enough functionality to let Python users to perform useful modelling.

Key features
- wrapping of cable cell functionality
  - user-defined explicit compartmentalisation not supported
- `single_cell_model` abstraction unique to the Python wrapper that simplifies developing and testing single cell models
- one-dimensional cell builder for Python wrapper that simplfies building cells that 
  - in and of itself limited in scope, but a very useful example of mapping a richer morphology builder onto `sample_tree`s.
- parsing of region and location expressions from strings
  - implements a generic s-expression parser that we can use for other purposes later