Update stormevent.py

[naingeet]

updating tide datum to 'msl' in function coops_product_within_isotach

[naingeet]

updating tide datum to msl

[SorooshMani-NOAA]

It makes sense to have some value as the default since in the COOPS API documentation it says (emphasis mine):

Datum is mandatory for all water level products to correct the data to the reference point desired.
Does not apply to Air Gap data, which is only provided relative to a fixed reference point on the bridge.

And most of the times we're interested in water level products.

@naingeet can you please make sure all the test go through? We format the code using black

[SorooshMani-NOAA]

Since there hasn't been any updates on this request I close it for now