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

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

Add new params 'largest_dims_length' to determine if Start() should examine all the files to get the dim or not

Add one 'if' condition to ensure the first file of each dataset exists. If not, metadata is assigned to NULL.