add method to fetch subsubgroups using path

[AntoineRenaud91]

"netcdf::File" and "netcdf::Group" now have two new methods called "group_from_path" and "group_mut_from_path" which make use of the new function "netcdf::groups::group_from_path" to fetch directly a deeper subgroup without walking down the subgroups tree and having to manage all group instantiations. It using the linux path format for folders e.g. "path/to/my/folder"

BEFORE:

let file = netcdf::open("my_netcdf.nc");
let group = file.group("my_group").unwrap();
let subgroup = group.group("my_subgroup").unwrap();
let subsubgroup = subgroup.group("my_subsubgroup").unwrap();

AFTER:
let file = netcdf::open("my_netcdf.nc");
let subsubgroup = file.group_from_path("my_group/my_subgroup/my_subsubgroup").unwrap();