Add variable mapping of read_tmy3

[AdamRJensen]

Is your feature request related to a problem? Please describe. This PR proposes that a map_variables parameter be added to the read_tmy3 function. Additionally, the current rename_columns parameter (which removes the units from the column names) should be deprecated. See #714 for a discussion on the topic.

Describe the solution you'd like A map_variables parameter should be added (defaulting to None), and if specified as True then it should override the rename_columns parameter and map the column names to standard pvlib names. A deperecation warning should be added stating that the rename_columns parameter will be retired starting in pvlib 0.11.0 - the deprecation warning should be silenced if map_variables is specified as either True or False.