Unify some of the backend queue methods.
Clean up how kernels interact with queues.
Permit kernels to take keyword arguments at runtime.
Permit pointwise kernels to take arguments at runtime.
Fix an accounting bug in the RK45 stepper.
Remove the little-used DOPRI5 RK stepper.
Refactor the time-step integrators/steppers.
Make the degrees of polynomials available in the polynomial bases.
Add a matrix copy kernel to each backend.
Take advantage of Matrix.nbytes in the backend.
Permit matrices to alias other matrices.
Simplify some constructors in the backends.
Add a basic exponential solution filter.

Syntax is:

  [soln-filter]
  freq = 1
  alpha = 14
  order = 16
  cutoff = 3

where freq is how often (in terms of # of time steps) to apply
the filter; alpha sets the strength; order sets the order of the
filter and cutoff is the cutoff polynomial degree below which no
filtering is applied.
Remove an unused import.
Fix MPI matrices.
Simplify views.
Merge branch 'feature/filtering' into develop.
Cleanups.
Disallow direct exponentiation when evaluating expressions.
Add support for time- and position-dependent source terms.