Implement "memory" resource

[sjdv1982]

Right now, the only resource that is really internal to Seamless is "ncores" (see "multi-core-transformations" test). "memory" should be added as well:

• Use bytes2human to parse the user specification
• Add it to Seamless itself with a lock mechanism. Something like: wait until enough memory is available, then acquire the global memory allocation lock, then decrease (allocate, claim) the available memory; then release the memory allocation lock. Have a concept of max memory, fail if that is insufficient. Also, for nesting, implement knowledge of the memory claimed by the parent process (unlike cores, don't release it when calling a child)
• Add it to the mini and mini-dask assistant
• Optional: add it to Dask as a Dask resource. Rather dangerous (may sliently hang) until my Dask patch has been created and accepted.