Added hydrotools.R

hydrotools.R contains functions and constants for:
- acre-feet to cfs-day conversion
- simple water year date manipulation
- error functions common to water resources:
a) excelR2 - not recommended, analog to excel's R2 measure
b) RMSE
c) nash sutcliffe model coefficient with alternative model
- Common plot breaks for flow-frequency curves:
a) flowBreaks for log10 scaled axes
b) probBreaks for 'normal probability' axis
Merge branch 'master' of https://github.com/eheisman/DSS-Rip