Add running multiple directories

[JCGoran]

Work on fixing #118.

Changes

• runmodels can now compile mod files from multiple directories in isolation. Note that for existing models which have model_dir set to multiple dirs (like path1;path2...), those are still compiled together to be consistent with current behavior
• made model_dir a YAML array instead of a string. As stated above, any existing models that have model_dir set retain the same behavior, except now we are able to compile multiple directory groups (in isolation) instead of just one group, so we can set something like:

  model_dir:
    - 'group1_dir1;group1_dir2'
    - 'group2_dir1;group2_dir2;group2_dir3'

  and those 2 groups will be compiled independently.

Prerequisites

• merge formatting: run Black on project #115
• merge Add .gitignore #116

[nrnhines]

I'm in favor of this. Multiple groups, each with potentially multiple directories seems general enough to cover all cases for building the nrnmech libraries. The only thing I wonder about is if the concept of multiple groups should be replaced by a hierarchical level just below the accession number so that distinct runs for each group can be performed. But that can be deferred to the indefinite future since it potentially involves modification of nrnivmodl. I've lost track of the capabilities and limitations of nrnivmodl and its consistency with nrnivmodl-core. I do know that the last time I changed nrnivmodl it allowed multiple directories and multiple specific file.mod paths allowing spaces in path names (but nrnivmodl_core did not).