Improved selection of default Python install path. (#1277)

* Docs: emphasised the option of having pip install a local copy of the Arbor source.
* CMake: default Python library install path to `${PYTHON_EXECUTABLE} -c "import sysconfig; print(sysconfig.get_path('platlib'))"`. Fixes #1100.
* CMake: remove ARB_PYTHON_PREFIX option, added ARB_PYTHON_LIB_PATH option. We cannot know in general where in the prefix the packages are expected by Python.