method to estimate base request size #263

A method was added to "OcgOperations" called "get_base_request_size".
This method will return a dictionary estimating the requested data
size in kilobytes. Docs updated.
allow file output without auxiliary files #260

Added overwrite capability to converters. This allows env.OVERWRITE
to operate as expected without output files in an output folder.

Closes #260.
Merge branch 'master' into i238-icclim