Add LogConstant time stepper

[agiorla]

Description of the enhancement or error report

Add a TimeStepper class that would set the time step so that it remains constant in the logarithm space.

Rationale for the enhancement or information for reproducing the error

Logarithmic time steps are useful for the modeling of long-term behavior of materials or structures, notably in concrete creep or durability.

Identified impact

(i.e. Internal object changes, limited interface changes, public API change, or a list of specific applications impacted)
New TimeStepper class to add, nothing else changed.

[dschwen]

@agiorla , you can already jury rig this with PostprocessorDT (using a FunctionValuePostprocessor and a ParsedFunction). But a dedicated class might not be a bad idea. I'd vote for creating an actual FunctionDT (the current one is a misnomer because it does not take a Function as an argument)

[agiorla]

I've actually already written that class for a MOOSE-based application, but we thought that it might be more useful to have it directly into MOOSE instead. I'm currently in the process of reintegrating that class into MOOSE. Also, I think that it is a good idea to have that as a separate class rather than an (improved) FunctionDT, just to simplify the user's task when writing input file (no need to define an actual function) [that said, an improved FunctionDT might be a good idea, but that is probably a separate issue?]

[permcody]

I guess we'll see what your class looks like when you post it. If it's a simple analytical function then we should probably just make a new/different FunctionDT. The timesteppers need to be cleaned up anyway and we have a few PRs that worked toward this but haven't been merged. One at a time is fine with me as well.