make ISRM explicit input

[lkoolik]

Update the code so that the user can input a custom ISRM file. This update should do the following:

• [x] Preserve the CA ISRM as the default
• [x] Update the control file to have an ISRM line
• [x] Add a few checks for the ISRM file
• [ ] Confirm that calculations still work with the new ISRM

[lkoolik]

Additional necessary steps (may add as I find them):

• [x] Update ISRM object (isrm.py) to add the necessary files to the file path