LogFile with some features

[apeltzer]

make nice looking log with stuff written currently to stdout ("echo" etc.)

[apeltzer]

Maybe in workflow.onComplete?

[d4straub]

Going to try out log.info as in https://github.com/nf-core/rnaseq/blob/master/main.nf line 539

[d4straub]

still to do:

• error message if dada_trunc values are too small in process dada_trunc_parameter
• warning if rarefaction depth is too low in process diversity_core

[d4straub]

done for dada_trunc_parameter

[d4straub]

done