doc

- added hacked version of the GNU matheval library to evaluate parsed
  functions efficiently ("a la getdp", but faster): I added a new
  directory (gmsh/MathEval) with that code, as well as a new configure
  option and a new define (HAVE_MATH_EVAL)

- generalized the plugin option code to accept arbitrary string options

- new plugin Plugin(Evaluate) that can evaluate arbitrary functions on
  scalar views (useful for example to create background meshes)

PS: Everything (octree + function evaluation) is now in place to write
a general plugin that can make any operation between arbitrary
views...