rebase
add constraints tests
fix linear attribute
remove an erroneous  import statement
Address comments by @iguananaut; add m ore tests
Rebased
remove a commented line (and start a new travis build
Revised the list of models with constraints. Also converted rotations classes to using the new Parameter object (somehow missed this the first time).
Merge pull request #1243 from nden/refactor-constraints

Refactor constraints
Add latex and unicode formats to Angle.to_string()
Add way to only include some of the sexagesimal fields.
Fix some failing tests