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.

Bugfix for Start() parameter 'path_glob_permissive' and write the explanation in FAQ and Start() header.