adding a stable way to define ISISROOT

[michaelaye]

The line:

ISIS_ROOT = os.environ.setdefault('ISISROOT', '/usgs/pkgs/isis3/isis')

in pysis.env is all what controls where to find ISISROOT.
I have found this very fickle in trying to make things work across several conda environments (which is now the default install for ISIS3), so I propose to add functionality to pysis.env.py to get the paths from a config file in the users' home.
Currently, this file on my side simply looks like this:

$ cat .isisroot.toml                                                                        (isis3)
ISISROOT = '/Users/klay6683/miniconda3/envs/isis3'
ISIS3DATA = '/isis3/data'

and I'm using the toml format (https://github.com/toml-lang/toml).
I'm thinking about a few approaches how this fluently could be combined with an existing os.enviroment variable, but let's discuss these details in my PR I gonna start later this week after my current meeting is over (where, of course, I needed pysis to work and it didn't ;) ).

We could also have this config file to be created after the current ISIS3 install, it's simple enough. But much more stable against environment variable manipulations caused by switching conda envs (admittedly, that's the whole idea of conda, obviously).

[Kelvinrr]

Should be simple enough to prefer a config over an env var (config doesn't exist, back up to use an env var). But what do you mean by "We could also have this config file to be created after the current ISIS3 install"? As in make ISIS generate a pysis config file? That is how I am understanding it which I don't think would be ideal.