Add Data checking function gbm.precheck

[SimonDedman]

Takes samples and vector of expvar & resvar columns. Produces:
Scatterplots of expvars
LM plots
Corr plots
spatial autocorrelation?

Do it as a markdown doc to produce a webpage, add text to advise people what to look for and fix.

[SimonDedman]

Also give option to add a random variable here, if the (an) outcome of gbm.precheck is an edited samples and/or grids.

Possibly check samples/grids colnames match

Possibly check factorial variables levels are in the correct order