WRTDS Enhancements

[jds485]

I've modified several of the EGRET functions related to WRTDS. Some of these enhancements may be useful to others. They include:

1. new function parameters to specify the resolution of the surfaces (number of time steps and flow steps).
2. option to include a quadratic term for log flow in the WRTDS survival regression.
3. option to plot surface contours for all of the regression parameters, not only the mean concentration and the model error.
4. a new function to make predictions from the WRTDS models within the estimation timeperiod, given tables of the parameters, flows, and dates.
5. a new function to fill in NAs in generated tables by inverse distance interpolation between neighboring cells in the table.

The changed functions include:
modelEstimation
estSurfaces
surfaceIndex
runSurvReg
run_WRTDS
estCrossVal
plotContours

I tracked all changes on my local machine in git, but I'm not sure how to share the git history with you. It may be simpler for you to do a git diff for each function to see what changes I made. I've attached the file of modified functions as a .txt file (GitHub Issues doesn't support .R files, so I can send the R script by email if you ask). Let me know if you have any questions.

WRTDS_ModifiedFunctions.txt