Fix edge cases

[rlbyrne]

This cleans up a bunch of minor bugs that broke things in recent runs:

• properly detect npol if no polnames in filenames
• handle descending time order in time resolution calculation
• Always use the vis_sigma calculated by FHD if it is available
• Only use the file basename when parsing polarizations out of filenames
• properly handle fully flagged frequencies in cube image plotting
• Fix a bug when we only have one file (no even/odd split)
• Properly handle a null pointer in for vis_noise in the obs structure (happens with only one time).
• Fix a bug in beam integral handling for single times (no vis_noise in obs structure), also generally clean up beam integral handling.