NF: tsdiffana option to save results volumes

Add option for tsdiffana to be able to save the results volumes created
during the analysis.
BF+TST: test+fix bugs for PCA on reordered images

The `screen` and `write_screen_res` functions were assuming time-last
ordering in a couple of places.  Test and fix.
WIP: deprecations and refactoring for nipy_diagnose