caiman.paths.memmap_frames_filename() to build paths in $CAIMAN_TEMP by default

[pgunn]

Addresses #1467 (first concern); this will address another case where memmap files can be made in the demo dir (in this case done by demo_seeded_CNMF)

We'll need to test the existing demos carefully to make sure this doesn't break anything, although if it does it'll be because the existing code is calculating paths rather than using the return value of the function that builds them, so it'll be good to fix that.