Do not auto-prune instance types if there are too many

[cartalla]

I was previously only allowing 1 memory size/core count combination to keep the number of compute resources down and also was combining multiple instance types in one compute resource if possible.

This led to people no being able to configure the exact instance types they wanted.

So, I've reverted to my original strategy of 1 instance type per compute resource and 1 CR per queue. The compute resources can be combined into any queues that the user wants using custom slurm settings.

I had to exclude instance types in the default configuration in order to keep from exceeding the PC limits.

Resolves #220

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