Extend global basis tests

This extends the global basis check checkBasis() by incorporating the check for zero-basis functions and adding a check for continuity of basis functions across intersections. The continuity check first matches inside and outside DOFs on an intersection via their global indices. Then it performs a local check which can be customized:.

• You can enable a continuity check by passing EnableContinuityCheck() to checkBasis().
• The default is checking if the norm of the jump is zero (this is simple continuity).
• By deriving from EnableContinuityCheck and overloading localContinuityCheck() this can be customized.
• A simple form of customization is to implement localContinuityCheck() using localJumpContinuityCheck() which allows to compute other pointwise jump-expressions like, e.g., the normal jump.
• Full customization is possible by a hand-crafted implementation of localContinuityCheck(). This e.g. allows to check continuity in edge midpoints only.
• A checking for the normal jump is implemented in EnableNormalContinuityCheck (but not tested so far).