Users not able to choose appropriate memory for different partitions

[wwarriner]

Max memory per partition is 128GB per CPU, so users can't request 1 CPU and 1024GB on largemem, for example. We might want to consider restricting memory selections based on the partition chosen. It isn't clear how to make this work in a fully-automated way (i.e. using scontrol, but I imagine the solution to this may be related to the complete solution to #86.