doc changes #183
disconnect source metadata from modified metadata to allow request datasets to be reused - closes #259

The source metadata dictionary is now deepcopied prior to passing
to field. The same occurs for the variable-level metadata.
Merge branch 'master' into i238-icclim
mark - before chaning get_output_units to set_output_units
mark - before adding metadata manipulations to icclim library
mark - before testing global attributes
mark - all tests working