Variable `mpich` is not defined

[efaulhaber]

This variable is only defined here https://github.com/Azrael3000/tmpi/blob/f5a0fd8848b5c87b301edc8a23de9bfcfbd41918/tmpi#L11-L13

and doesn't exist here https://github.com/Azrael3000/tmpi/blob/f5a0fd8848b5c87b301edc8a23de9bfcfbd41918/tmpi#L108-L112

or here https://github.com/Azrael3000/tmpi/blob/f5a0fd8848b5c87b301edc8a23de9bfcfbd41918/tmpi#L237-L241

This causes tmpi to fail with MPICH, as --oversubscribe doesn't work.