Allow ColumnMapper Known Map strings to be supplied directly to loader functions

[dougbrn]

Feature request Known maps offer some nice convenience for users to just create a column mapper object from a string (e.g. ColumnMapper().use_known_map("ztf")). It would be even better to just allow users to pass the string as an input to functions like this: from_parquet(column_mapper="ztf") instead of needing to create a ColumnMapper object up front.

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[dougbrn]

Add some kind of output that lets the user know what mappings were assigned