Modify WarmUp Formula

Hi, my friends and I love the warmup-reps app, but noticed we were spending an awful lot of sets doing just the bar. I have modified the findWeight and roundDown functions to provide, what I feel, is a smoother progression to your working set. 

Basically ((WorkWeight - Bar) * Multiplier ) + Bar) as opposed to (WorkSet * Multiplier). This also eliminates the need to check if the warmup weight is less than the bar. As long as the workset is over the bar weight, things should be golden. 

This is my first pull request, apologies if I borked it up badly.