Handle multi-segment SortingAnalyzer

[alejoe91]

Currently, multi-segment support is not fully implemented.

Main friction points:

• In NWB, multiple segments will be written as multiple electrical series, so we would need to be able to point to multiple ES for multi-segment
• Sorting object from units table should also be loaded as multi-segment
• Valid unit periods: periods for different segments should be offset by segment start time in save/load functions

@h-mayorquin anything else comes to mind?