Should pycytominer generate additional metadata columns by default?

[gwaybio]

In #129, @niranjchandrasekaran proposed that pycytominer create a metadata column, called Metadata_fields_count, that will be added to all future pycytominer-derived aggregate profiles. This represents the first time (IIRC) that pycytominer is actively creating a metadata column, instead of passively using those provided in the platemap files.

Adding more default metadata columns helpful to all profiling experiments in beyond scope of #129, but we can track such columns in this issue.