Improve the process requests for tools in the pipeline

[SPPearce]

Description of feature

Samtools pileup has the config process label of process_medium, but is not parallelised at all. It requests 6 cores and 36GB of RAM by default, but doesn't need the cores. If the RAM requirement could also be decreased by default that would also be a good idea.

These are the modules that use process_medium: https://github.com/nf-core/sarek/search?q=process_medium