allow user to input list of numbers for stretchednmfapp/parser

[aajayi-21]

Currently, in stretchednmfapp/parser, the user inputs an integer specifying the number of signals for the NMF decomposition which is used in the alogirthm.

Desired behavior would implement a use case where the user would supply a list of numbers.

[john-halloran]

Closed by #187. We have this, I think. If the issue is referring to a grid search over a range of components numbers, we don't have that, but we can make a new issue where it's more explicit.

[sbillinge]

closed as implemented