Revise the checks for depending dimension list names; Add explanation of how to use values() to define dependency relation.

Stop if merge_across_dims and split_multiselected_dims are both used but this file dim selector is multi-dim array

Fix the time unit attribute retrieval if the format is YYYY-M-D. Also, split_multiselected_dims is changed to FALSE if it is useless.

If the name of return_vars is the synonmin of inner dim name, change it back to inner dim name. Unit test added.