Load multiple datasets from single ORSO file

[bmaranville]

@acaruana2009 wants to be able to save all the reduced data for a single sample condition.

This includes possibly multiple incident angles, and for each incident angle includes multiple polarizations.

It is proposed that load4 is modified so that it can read multiple entries, group them properly by incident_angle and polarization, and create a list of PolarizedNeutronProbe objects for each incident_angle

It is not clear if it will make more sense to save all entries at the same level (but they are distinguished by the values of the incident_angle and polarization stored in the ORSO metadata).

We may want to add an unambiguous grouping tag instead of relying on incident angle, which relies on (problematic) floating-point comparison (jitter in the value of the motor? other floating-point fails?)