Updates to fret-diagrams

Updates to fret-diagrams

Add new orientation opposing-landscape as requested by
    John Sellers
Revise orientation code so 'normal is always a default (in
    the else clause of a cond)
Adjust the origin of the fret diagram so that the top fret
  on the diagram is always at the origin
Clean up calls for fret-diagram-details so that merge-details is always
  used.
Clean up text stencil alignments
Fix bug in thick top fret
Rationalize code so all code is written in terms of fret-coordinates
  and string-coordinates, with a transformation to stencil (x-y)
  coordinates depending on orientation.
Split regression tests into multiple files, with a texidoc
  string for each file