Fix a bug when evaluating periodic boundary conditions.
Adding system of ODEs case and avoid IF statement.
Modification suggested by Asgeir.
Minor modifications and add documentation.
Adding a test.
Merge branch 'development' of ssh://github.com/chebfun/chebfun into fix-periodic-nonlinear
Fixing typos, and clarifying comments.

Also ensuring different signs of results for evaluting BCs don't
cancel out by squaring the results straight away.