Added test for DG basis classes.

Only 2d so far.
Merge remote-tracking branch 'upstream/master' into dg-improvements
Refactored limiter functions.

The two limiter functions were very similar, and were unified in a single
method, with a bool argument to choose method variety.
Added functionAverage() method to DGBasis classes.
Use functionAverage() instead of direct coefficient access.

This fixes the issue with limiters on multilinear basis.
Add methods totalFlux() and minCornerVal().

Also started refactoring applyMinUpwindLimiter() using the
added methods.
Refactored applyMinUpwindLimiter().
Added evalFunc() method.

This public method is not virtual, and implemented in the base class using calls to
the virtual methods. Not yet used by the DG solver.
Merge remote-tracking branch 'upstream/master' into dg-improvements
Merge pull request #120 from atgeirr/dg-improvements

Further DG time-of-flight improvements