Change default fillna to be false and allow configurable fill value

[jtmiclat]

Discussed in #62

^{Originally posted by mosesckim June 30, 2022}
Noticed default is to fill NaNs with zero after aggregation; this might make it difficult to identify original NaNs if there are actual zeros in the aggregation.

https://github.com/thinkingmachines/geowrangler/blob/master/geowrangler/vector_zonal_stats.py#L205

Also, a suggestion would be, in the case fillna option is set to True, make the replacement value (0 in this case at the moment) a variable users can input (e.g. -1, etc.)

Thanks!
Moses

[butchtm]

I'm setting this a good first issue for new contributors!

[butchtm]

Implemented in #94