Untested: reduced-memory reading of ptv_is files
Low memory-usage ptv_is reading, debugged.
Allow filtering short trajectories on read, to avoid clogging the memory with unused trajectories.
Save sequence configuration + tests.
Save interpolant configuration + tests.

The read_interpolant function was already present, having been committed previously by mistake. The tests resulted in a small fix making the parameter applicable also for the inv method
Merge branch 'ptvis_mem' into working_branch

Added 'traj_min_len' as a saved key with override.

Conflicts:
	flowtracks/sequence.py
Fix: wrong file opened by sequence reading test, found by Alex.
Small fix to prevent bombs on logarithmic plots.