- Complete revamp of methods in class matrix. Some redundant (and poor)

- Complete revamp of methods in class matrix.  Some redundant (and poor)
  implementations of elimination schemes were thrown out.  The code is now
  highly orthogonal, more flexible and much more efficient.  The function
  lsolve() now falls back to matrix::solve(), which in turn calls different
  elimination strategies.  matrix::fraction_free_elim() was thrown out for
  good.  matrix::pivot() has now two integer arguments (ro and co) to allow
  pivoting when eliminating to echelon form.
- Some new checks to guard against regressions in the new matrix code.
- Some minor bug-fixes in documentation.
- Bumped up version from 0.6.3 to 0.6.4.
- ...and the usual changes triggered by other changes.