Added functionality for proper treatment of exactly parallel or perpendicular
layers in the stack of operators.

- added a function 'aggregate' which pre-processes the stack of operators, and
  amalgamates wherever possible, prior to effective splitting calculation.

- added appropriate tests into the test suite.
Merge branch 'master' of ssh://majorite/geophysics/walker/git-repos/msat
Make a start on the updated release notes.