Allow user specified dtypes when creating a DataFrame from map about explorer

[cigrainger]

@dgreiss I'm not convinced by this one, which I think I mentioned in Slack. The reason you can specify dtypes when reading from a csv is that the csv reader typically only reads a certain number of rows to infer the types, so it may get it wrong. When using DataFrame.from_map/1 you either pass in series directly or lists go through Series.from_list/1 which traverses the entire list.

[cigrainger]

I'm going to close this as wontfix, but please feel free to make a counterargument to the above if I'm off base here. Thanks!

[dgreiss]

Yep that makes sense thanks for clarifying. My thinking was that by providing the dtypes option, you would be manually casting the types and you would not need to traverse the entire Series to get the type info. If the user input was supplied incorrectly, it would raise when casting.