Add support for reading from a group

The function `read_var` only support reading variables in the root group. If there are nested groups, then `read_var` does not work. This is also a little tricky, since everything that is needed to construct an `OutputVar` might not be in the group. For example, according to the [netcdf documentation](https://docs.unidata.ucar.edu/netcdf-c/current/groups.html), dimensions defined are available to all parent groups. This mean that if a dimension is not in the group, then one must recursively search for the dimension in the parent groups. Using `NCDatasets.jl`, this is available by `NCDatasets.group(nc, group_name).parentdataset`.