Added function to plot a cutplane from tesseroids

If passed a surf (output of myv.tesseroids) will get the data source
from there. Else, converts Tesseroids to TVTK.
Separated the part that converts Tesseroid to a TVTK data source for
reuse.
Merge branch 'master' into block-inversion
Merge branch 'master' into block-inversion
Merge branch 'block-inversion' of https://github.com/birocoles/fatiando into prisma
Merge branch 'prisma' of https://github.com/birocoles/fatiando into prisma
correct the functions kernelxx - kernelzz