Override Namelist get() to handle cogroups
Override Namelist get() to handle cogroups

The get() function was using the lower level `dict()` subroutine, which
is now an issue since we are overriding __getitem__ to accommodate
cogroups.

This patch re-implements get() so that it uses the Namelist dunder
methods.

Thanks to GitHub user @chunderbunny for reporting this issue.

marshallward/f90nml,marshallward/f90nml