v1.1 updates

[dpesce]

This pull request includes a number of modest capability improvements and other quality-of-life updates.

A couple of issues are closed with this update:

• Closes Add a priori flux density calibration example script #20. There is now a calibration module that is optionally included with ngehtsim by specifying [calib] during installation. This module includes functionality for carrying out a priori flux density calibration and writing dlist files, and an example script is included.
• Closes Todo: add night-time observing constraint #23. The optional argument "flagday=True" can now be passed to the make_obs functions, which will produce datasets that only contain nighttime observations. Alternatively, users can have more fine-grained control over the uptimes at each site by passing a "station_uptimes" dictionary when initializing an obs_generator object. An example script is included that shows how to use this functionality.

In addition to closing the above issues, there are a handful of other notable updates:

• A number of new telescopes have been added to the database, including EAVN and VGOS stations
• The weather database has been updated to include dates through the end of 2023
• The code will now yell at you if you try to pass unrecognized settings during obs_generator initialization
• SYMBA export functionality has been updated to reflect changes to MeqSilhouette
• Substantial improvements to the unit tests and the addition of code coverage tracking