add support for training own .RData file

[noahfl]

should just be a matter of adding another environment variable and updating some lines of code, but I'll have to wrap up the fix issue before getting to this

[shen4brains]

@noahfl As an update to this issue, a more generic way of creating the .RData file where it takes a .txt list of directory names would be a nice enhancement. See how it is currently hard coded for ADNI3 here: /liberatrix/mcintosh_lab/kshen/ADNI3/create_RData.sh