Add support for reading variables in groups.

[ghweber]

Add support to read variables that are not in the root group but instead a group. (Replaces #798)

Refer to variables by <groupname>/<varname>' in.mfc` file.

Limitations:

• Not sure if it is ok to cache cf_parent_id or if a group ncid can change when a file is closed and re-opened.
• Currently only supports variables in top-level groups. Need to add recursion to support groups at arbitrary depth in hierarchy.
• No support for groups in teca_wrf_reader, yet. (may not be needed according to Travis)