Test if FD approximation of AV derivative term can improve things (!78)

* IntegrationTests: print Inf norm if mismatch is found

* update refdata burgers_sine_avmda

* update refdata burgers_sine_avdb

* update refdata burgers_sine_aventropy

* update refdata burgers_sine_avtci

* fixup constructor call

* more examples

* update output times; fixup gitignore

* utilize Sandbox to parse expressions from parfiles

* fixup ScalarEq bump initial data

* add examples folder with parfiles that are not ref tested

* specialize timestep for AV scheme

* implement 1st order fd approx for derivative of regularization term

* enable callbacks

* add prms field to ScalarEq's project struct

also add av_derivative_scheme parameter to ScalarEq project
to be used subsequentely

* simplify dispatch of rhs!