Use FOURCOLLOC class for periodic eigenvalue problems.
Two changes.

1. Pass correctly the prefs to linop/eigs.
2. Add tests.
Merge remote-tracking branch 'origin/development' into feature-eigs-periodic
Merge remote-tracking branch 'origin/development' into feature-eigs-periodic
Several modifications.

1. Add a adjustPref method, used by both SOLVEBVP and CHEBOP/EIGS.
2. Modify the tests to check if the solution is of the right tech.
Changes suggested by Asgeir.

Split adjustPref into two methods: clearPeriodicBC and determinePref.
Method renaming.
Rename determinePref -> determineDiscretization. And changes on inputs/outputs, and
check if fourcolloc outside clearPeriodicBCs.
Merge branch 'development' of github.com:chebfun/chebfun into feature-eigs-periodic
Adding documentation.
Missing from previous commit.
1. Delete the TODO in CHEBOP/EIGS.
2. Documentation in CHEBOP/DETERMINEDISCRETIZATION.
Fixing a typo.
Merge branch 'feature-eigs-periodic' of github.com:chebfun/chebfun into feature-eigs-periodic