Cutoff outstanding values in 2d-plots

Prevent blank areas if one use custom scale bounds
Add `col' argument to 2dplots
Document the  `col' argument
Add at argument to wcor
Add `zlim' argument to plot2d
`at' can be a list now
`fill.color' may be a list now
Merge pull request #169 from eodus/plot2dcolors2

Various small fixes for 2D SSA