OPSIM:1050 Update m52snr and add y-band parallax metric

[rhiannonlynne]

Many of the commits in this PR are updating line length in doc strings, moving a bit more code to snake_case from camelCase, removing unnecessary or unused code (imports or values that were not used), updating super() calls to remove the / self arguments, or standardizing metric API ("Metric" at the end of metric classnames, metric_name doesn't need to be a kwarg if the default is the same as the class name).

The meaningful changes though are:

• addition of fwhm_500 to the ModelObservatory attributes (this was helpful for a test of m5 difference basis functions, when changing the input seeing .. in order to track what the 'original' value was).
• update FO plot to use sky and n_visits kwargs correctly in the summary metrics printed in the legend

and the bigger changes:

• updating m52snr to use a more accurate SNR formula
• updating the astrometric precision estimate to also include the systematic floor (instead of having to add this separately in the returned values) -- moving it to the function should make it easier for other users to remember to include this
• adding a y-band parallax uncertainty metric to the astrometry_batch

[yoachim]

Few minor things.