Many new metrics and improvements to graphics2

New metrics:
* '-m marginal': Mean probability of thresholds
* '-m marginalratio': Obs frequency to marginal probability
* '-m bss': Brier skill score
* '-m bsrel': Brier score, reliability
* '-m bsres': Brier score, resolution
* '-m bsunc': Brier score, uncertainty
* '-hist': Show histogram of values
* '-ct': Choose min/mean/median/max of metric
* '-m invreliability': Show reliability diagram for quantiles

Improved metrics
* '-m pithist': randomize PIT values on boundaries for variables
   with discrete mass (e.g. Precip)
* '-m ef': Improved for x=0 for Precip
* Renamed '-m brier' to '-m bs'
* Allow multiple thresholds in '-m reliability'
* Propagate '-leg' to all plot types