Introduce OPM_UNUSED as a portable wrapper to __attribute__((unused))

This works pretty much the same way as DUNE_DEPRECATED (without message).

Thanks to Elias Pipping for implementing this.

dune-common SVN revision: r6556
add tridiagonal matrix

this class can do both, more and less than dune-istl's BTDMatrix. more
because it supports entries on the lower left and upper right, less
because it does not support a block structure. The primary motivation
for this class were the spline classes which for which the former
feature is required to implement periodic splines and the latter is
not necessary...
add functions to analytically invert polynomials up to degree three
introduce classes for monotonic, full, periodic and natural cubic splines