Add methods totalFlux() and minCornerVal().

Also started refactoring applyMinUpwindLimiter() using the
added methods.
added: license
Merge pull request #124 from akva2/add_license

Add COPYING file with license
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