Add DofMap::(un)stash_dof_constraints()

Adjoint-based sensitivity calculations can require us to calculate
residuals without applying constraints, and this seems to be the least
modularity-defiling way to let us turn them off and back on quickly.
adjoint_qoi_sensitivity bugfix for adjoint BCs

With heterogeneous adjoint BCs we can't constrain the residual vector
and still expect to accurately calculate adjoint-weighted residuals
with a dot product.
Fix lift_func errors in adjoint qoi sensitivity

Choosing the right BCs and not making a sign error might be helpful...
Use API properly

enforce_constraints_exactly took a pointer to allow for a default
NULL; but enforce_adjoint_constraints exactly takes a reference...
has_heterogenous_adjoint_constraint changes

New method has_heterogenous_adjoint_constraints tests for any such
constraints.

Old method has_heterogenous_adjoint_constraint new returns the
constraint rhs Number rather than bool; this should be semantically
backward-compatible with the old system unless someone was manually
setting a zero constraint and expecting that to count as heterogenous.
Integrate regularized flux QoIs in FEMSystem
FEMSystem integration of heterogeneous qoi_deriv

This is consistent with the new assemble_qoi behavior
Selectively apply liftfunc and constraints

We need the liftfunc terms to be left out of the RHS for adjoint
solves; we need QoI derivative vectors to be left unconstrained when
using them for dot products in sensitivity calculations.
Fix a bug Vikram caught in FEMSystem qoi derivs
Implement our own isnan() if necessary

Even if we don't have C99 isnan() we ought to be able to rely on IEEE
isnan.
Merge pull request #376 from libMesh/isnan_fallback

Implement our own isnan() if necessary
Update references

* Berger, 2013
* Sartori, 2014
Update traffic.
Don't hard-code true solutions in Jacobi unit tests.
Add QCONICAL to the list of rules supported by QBase::build()
Remove redundant logic in Jacobi quadrature unit test.
Add unit tests for 2D quadrature on TRIs and 3D quadrature on TETs.
Updating Corsini, 2014.
Add QCLOUGH to the list of Triangle quadrature rules to test.
Merge pull request #377 from libMesh/quadrature_unit

Add unit tests for quadrature on TRIs and TETs