Fix importance map creation

Modify Jesse's importance map code so that there is slightly more
abstraction. Add flag <generate_importance_maps> to possible inputs
to run mvpa function. Add flag <xval_iters_for_imp_map> to determine
which xval iterations are reported in the importance maps. Improve
string handling.

Would be smart to clean up this code so there isn't so much redundancy
with the if feature selection portion.
automatically save average importance map

it remains unclear to me whether it is better ot use spm_mean or spm_imcalc, but I'm doing the latter in order to automate this process. In Mango the results look quite similar, although explicitly calculating the difference between the two images yields some discrepancy.
will merge to master
Add Rmds for figures for MacArthur Meeting

Not relevant to this branch :(