Update missing data in the cache of tractography.

Flexible computation of the cache. If some data are missing in the cache the update_info() method computes only them. This patch enable to include the computation of the dissimilarity matrix as part of the pipeline.
Main changes:
1. new function update_info()
2. moved the code for cache computation from 'load_full_tractography' in the new method
3. moved the check of exists file in 'save_info'