Elaborate on IgBlast and ChangeO installation in docs

[BrandenOlson]

It might not be obvious to the typical user where Change-O puts its scripts (i.e., ~/.local/bin), and sumrep makes use of a CHANGEO_DIR environment variable to run these scripts. Further, using ChangeO scripts to build the IgBlast libraries may entail cloning the repository, and building IgBlast libraries with some input/output names different from ChangeO docs will lead to issues for sumrep (e.g. in finding the germline directory). We should elaborate on this further in the docs to facilitate this installation process.

[javh]

Change-O installs scripts via the scripts argument to setuptools.setup(). When you specify the --user argument to pip, this means they end up in $PYTHONUSERBASE/bin. I don't know why it was decided to default this to ~/.local on linux and Mac OS (I always change it). See PEP370 for OS specific location details.

I'm guessing it'd be cleaner to check the PATH first (if you didn't pass --user to pip, then it'll install into the system location), then fall back to PYTHONUSERBASE/bin to automatically set CHANGEO_DIR.

Though, my feeling is that this is more of a normal python issue, rather than something sumrep needs to address. So, properly documentation is probably sufficient.