Refactoring of stiffness matrices

Stiffness matrix calculations have all been moved out of FiniteElement
to the StiffnessMatrixBuilder class.  Unit tests have been updated to
reflect this.
Linear3DBeam implemented

Although unverified (and with a failing test), the implementation of a
Linear3DBeam should now be complete.

Plus miscellaneous renaming of some of the other elements and the
associated stiffness classes and tests.