init should have outdir required argument

[kopardev]

Description of feature

init command should:

• require outdir argument , which is path to the output directory
• check that outdir does not exist and is writable
• create outdir as per

[kelly-sovacool]

@kopardev what do you think about calling it something like projdir since this would be equivalent to the nextflow $projectDir variable?
outdir is already a parameter in our nextflow workflows and I'd prefer to try not to confuse the outdir projectDir and workDir

(see also CCBR/CHAMPAGNE#219)