Update mechanisms loading using NEURON API

[anilbey]

Description

This PR uses the modern NEURON API in dealing with the mechanisms loading.
E.g. Bluecellulab previously was only checking for the x86_64, NEURON however supports a wider range of machine architectures such as: platform.machine(), "i686", "x86_64", "powerpc", "umac".

CHANGES

• remove load_nrnmechanisms from tools, NEURON does it more comprehensively (e.g. https://github.com/neuronsimulator/nrn/blob/e0290762790622291c4867dee25a7c13e0c2d326/share/lib/python/neuron/__init__.py#L323)
• use neuron.load_mechanisms instead of neuron.h.nrn_load_dll, load_mechanisms is more comprehensive
• add type_aliases.py to annotate NEURON specific types
• remove nrn_disable_banner: no longer applicable
• Addresses replace neuron.h.nrn_load_dll call with the high level load_mechanisms function  #20

BREAKING CHANGES

• rename expected env var BGLIBPY_MOD_LIBRARY_PATH -> BLUECELLULAB_MOD_LIBRARY_PATH
• the env var expects the directory containing the mechs dir (same as in NEURON). Before, it was expecting the .so file (which is platform specific).

TODO

Squash and tag since there's behavioural change in import_mod_lib.

[AurelienJaquier]

Very nice! Does this also work with NEURON 7? If not, is it ok to drop the support?

[ilkilic]

Looks great!

[anilbey]

Very nice! Does this also work with NEURON 7? If not, is it ok to drop the support?

Yes the function was there 6 years ago.

neuronsimulator/nrn@8ae0ca8

[anilbey]

I just changed the commit text and PR text, removing the mention of NEURON>=8