Rename `searchString` to just `search` or `filter`

[blaylockbk]

I think I'd prefer using the argument name search instead of searchString.

# Current
Herbie('2023-01-01').xarray(searchString="TMP:2 m")

# proposed
Herbie('2023-01-01').xarray(search="TMP:2 m")

# or 
Herbie('2023-01-01').xarray(filter="TMP:2 m")

[blaylockbk]

I actually am liking the word "filter" here instead. I'll see if anyone else has comments on this.

[blaylockbk]

oh, filter is a built-in function. Can't use that.

I'll just replace searchString with search.

The reason is becuase search is really a regex search, not a string exact match.