Allow setting fixed frequency ranges to avoid in uvcontsub

[e-koch]

Adds an additional keyword entry for line definitions in config_definitions.txt:

exclude_freq_ranges_ghz: given as a list of list, this gives a set [low, high] frequency range in GHz to avoid when using uvcontsub.
For example:

line_product    hi21cm    {'line_tag':'hi21cm','channel_kms':0.42,'statwt_edge_kms':50.0}
line_product    hi21cm    {'fitorder':0,'combinespw':False,'lines_to_flag':['hi21cm'],'exclude_freq_ranges_ghz':[[1.42002671,1.42078479]]}

This is useful when multiple targets emit in the same transition along a line-of-sight (e.g., Galactic and extragalactic 21-cm HI). The example above sets a fixed v_rad+/-80 km/s avoidance window for Galactic HI.

This appears to working well for our current HI data, and should have no impact on other uses as the keyword is ignored when not present.

FYI @akleroy

[e-koch]

@thomaswilliamsastro mind having a quick look through? This is working in production for the LGLBS HI imaging.

I need to merge this soon for the next round of LGLBS processing, though.

[thomaswilliamsastro]

Looks good to me! Just noting this also seems to include numpy-related fixes and some CASA import issues

[e-koch]

I'm encountering no issues with my updates to the imports. Please ping if anyone does hit a problem related to these changes!