Use explicit memory units for LSF executor

[bentsherman]

Close #5182

LSF supports explicit memory units with bsub: https://www.ibm.com/docs/en/spectrum-lsf/10.1.0?topic=requirements-resource-requirement-strings#vnmbvn__title__3

It is much simpler to use explicit units rather than try to infer the defaults from the lsf.conf. However it would be good to verify that this syntax has been supported for a while.

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[bentsherman]

@d-callan please make sure that the generated job scripts in the tests look good to you

[d-callan]

yes, its beautiful. thank you.