Implement revision last before in SpikeSortingLoader and SessionLoader

[k1o0]

In order to keep analysis results consistent the BWM code should load data via ONE using the revision argument which finds the last revision before a given date. Currently the SpikeSortingLoader only allows the loading of exact revisions. Here's the suggested solution:

• Add argument to load_spike_sorting method to allow loading last revision before a given date
• Optionally rename the current 'revision' parameter to 'exact_revision' to prevent confusion with behaviour of 'revision' param in ONE load methods

[Edit] I think this is implemented by using enfore_version: spike_loader.load_spike_sorting(revision=MODIFIED_BEFORE, enforce_version=False)

The SessionLoader currently has no way to pass in a revision.

[bimac]

for now: leave spike sorting as it is, add revision