Fix errors that arise when self-calibrating multi-EB mosaics with spws that change across the EBs

[psheehan]

This PR fixes two issues that arise when presented with a multi-EB mosaic that has matching spws but with ids that are different for each EB. The two issues are:

• get_meanfreq was being called in a loop over EBs, but being passed the list of all EBs but given the spws for the current EB in the loop. If the spws don't match, an error is thrown. This is fixed by passing a single element list of the current EB in that loop.
• When making the single-field-equivalent PB image for mosaic sub-fields, only the first EB containing a given subfield is passed to im.selectvis, but the list of all spws for all EBs is passed to the spw parameter. I'm not 100% certain how selectvis parses this, but when the spw IDs don't all line up across all EBs, it causes a failure that prevents the .pb.tt0 image from being created. Then selfcal fails downstream when it tries to access this image to get image statistics. The fix is to make sure that only the spws for the selected EB get passed to im.selectvis.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
auto_selfcal/selfcal_helpers.py	60.00%	2 Missing ⚠️
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Files with missing lines	Coverage Δ
auto_selfcal/selfcal_helpers.py	62.88% <60.00%> (-0.01%)	⬇️

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