Enable selection option for work to do.
Enable choosing lltf or dctf work.
Merge branch 'master' into options

Conflicts:
	mfloop.py
Enable support for fetching work from gpu72 by GHz-days
Fixed off-by-one error in calculating GHz-days for existing work in worktodo.txt.
Optimized GHz-days calculation.
Changed long form of new command line parameters.
Corrected help for --ghzd_cache parameter.
Merge pull request #2 from MarkRose/options

Enable many gpu72 options