run_lems_with_jneuroml_brian2 surprisingly requires the path to the LEMS file to be in the current directory

[jonrkarr]

This is the problematic statement:
https://github.com/NeuroML/pyNeuroML/blob/development/pyneuroml/pynml.py#L1592

This statement requires the LEMS file to be on the Python path and more specifically in the current directory. This is surprising because this is atypical, and not required to execute LEMS files with pyNeuroML itself. Managing this is also complicated by LEMS files referencing other files.

A more flexible option to dynamically import modules is to use the importlib module. This would also enable this method to work on LEMS files whose names wouldn't be names of valid python modules (e.g., names with dashes).

Ideally, this method would also not rely on the extension of the LEMS file being .xml, and allow other extensions.

[jonrkarr]

I managed to get this to work, but it requires an unexpected amount of hacking beyond what's required to run simulations with jNeuroML:

• Change to a specific directory
• Set up the Python path in a particular way
• Locate the LEMS file in a specific place
• Set the exec_in_dir argument to a specific value