Set datum for `coops_product_within_region()`

[FariborzDaneshvar-NOAA]

I noticed that default value for datum in the coops_product_within_region() function in stormevent.py, is None which will return STND datum (see example plot below).

Users can set datum=NAVD or any other value from the following list depending to their use
datum_list=['STND', 'MHHW', 'MHW', 'MTL', 'MSL', 'MLW', 'MLLW', 'NAVD']

I would suggest changing the default to NAVD or MSL as these two are more commonly used than STND, and/or adding the list of possible options to line 446 .

@SorooshMani-NOAA, Let me know your thoughts. If you don't mind, I can make these changes.

[SorooshMani-NOAA]

@FariborzDaneshvar-NOAA I agree that MSL or NAVD88 makes more sense to have. Note that we're also in the middle of updating the searvey's data API for COOPS. Feel free to update the default and start a PR, but please keep this open since I'm already working on the new API.

[SorooshMani-NOAA]

@FariborzDaneshvar-NOAA in the COOPS API documentation it says

NAVD: North American Vertical Datum - Note! This datum is not available for all stations.

https://api.tidesandcurrents.noaa.gov/api/prod/#datum

[FariborzDaneshvar-NOAA]

@SorooshMani-NOAA Thanks! So how about using MSL as default?

[SorooshMani-NOAA]

I think it would be safer, let's go with MSL for now

[FariborzDaneshvar-NOAA]

PR: #90