remove_unconnected_nodes does not work for network types other than heating and cooling

[PGorzalka]

If calling UESGraph.remove_unconnected_nodes with network_type others, an error message
UnboundLocalError: local variable 'is_supply' referenced before assignment
cannot be avoided.

This may be intended behaviour, but the docstring lists all network types {'heating', 'cooling', 'electricity', 'gas', 'others'}. I would at least add 'others', and add the following after line 1654 of uesgraphs/uesgraph.py:

        elif network_type == 'others':
            is_supply = 'is_supply_others'
            nodelist = self.nodelists_others[network_id]
        else:
            warnings.warn('Removal of unconnected nodes is not supported for network type {}.'.format(network_type))
            return removed