update components of MCMC potential fxns to work with daily data

[drounce]

There are two elements that are hard-coded that will fail:

• mb_max_loss divides by 12
• t2_idx uses year and month as opposed to considering daily data

Suggested fix:

• use gdir.mbdata['nyears'] instead of hard-coding dividing by 12
• t2_idx can use the same format but instead of grabbing the value by the list using position [0] use position [-1] to ensure you get the last day of the month