ported Skin plugin to the new API

dimensions.

fix STL export of quads

call setVolumePositive() in all ::writeXXX() functions

- don't use 'this->' unless you really need it
- don't use 'using namespace std'
- don't put extra ';'s everywhere
- follow the style of the rest of the code for indentation, etc.

too hard to maintain

- replaced old 2nd order views with new adaptive stuff (still missing
  some function spaces for quads/hexas)

revert to old tetgen code for now: the new code is just too buggy

you can use the new code by configuring with --enable-tetgen-new

- fixed bunch of warnings

Added ability to "sacrifice" higher dimensional generators of
homology groups in order to compute lower dimensional generators faster.

Fixed all known issues with cell combining. Also greatly improved its
performance.