Pymwalib

* Removed Python 3.7 from CI
* Updated CI to use mwalib 0.16.3
* Updated Dockerfile to use mwalib 0.16.3
* Major refactor to sum-vcs.py example (and fixed several bugs with it)

* Requires mwalib 0.16.*.
* MetafitsMetadata->ra_phase_center_deg is now NaN not 0 when missing from metafits file.
* MetafitsMetadata->dec_phase_center_deg is now NaN not 0 when missing from metafits file.
* MetafitsMetadata->grid_name defaults to 'NOGRID' when key is missing from metafits file.
* MetafitsMetadata->grid_number defaults to 0 when key is missing from metafits file.
* MetafitsMetadata->sun_alt_deg where key is missing from metafits file, value is represented by a NaN, not 0.
* MetafitsMetadata->sun_distance_deg where key is missing from metafits file, value is represented by a NaN, not 0.
* MetafitsMetadata->moon_distance_deg where key is missing from metafits file, value is represented by a NaN, not 0.
* MetafitsMetadata->jupiter_distance_deg where key is missing from metafits file, value is represented by a NaN, not 0.

* Requires mwalib 0.15.*.
* Added dut1 to MetafitsContext

* Requires mwalib 0.14.*.
* cable_delays_applied is now an enum.

* Requires mwalib 0.13.*.
* RFInput->digital_gains are now a list of doubles as mwalib >=0.13.0 is also applying the division of 64 to restore the gains value correctly.

* Requires mwalib 0.12.*.
* Added new example to simply print tile info (examples/print-tile-info.py).
* Fixed bug where rfinput.index was not set correctly.