Added a method evaluating a GridFunction at specific points on a single element.
Fixed a bug in the choice of quadrature order for local operators.

(Only affected basis functions of polynomial order > 1).
Functions from dune_basis_helper.hpp now take a Dune basis object
as an optional parameter.
Added the function acc() for accessing elements of STL containers
with or without bounds checking depending on compilation flags.
Added spaces with bases composed of Lagrange polynomials of
user-specified order.
Added a function exporting a GridFunction to a Gmsh file.