Filter refactor

[navidcy]

After the ClockReplacesState #96 refactor we are now able to construct a module whose sol is an array of arrays of different types (see, for example, https://github.com/FourierFlows/GeophysicalFlows.jl/blob/96fbb1b7844da0aa35764769e487125ef4f4de5b/src/barotropicqg.jl#L167). This brings up the necessity to generalize the makefilter since at its current form it won't work with the BarotropicQG with U(t) as implemented in the branch above.

Actually, a better suggestion is to remove FilteredTimeSteppers all together and instead add the optional post-timestep application of a function poststep! to sol. This way the user can prescribe filters in the form of the solution within any module and then apply them, e.g., through

function poststep!(sol, params)
  @. sol *= params.filter
end

[glwagner]

Except it should probably be

function poststep!(prob)
  @. prob.sol *= prob.params.filter
  nothing
end

[glwagner]

Functions that are applied at the end of a time-step are sometimes referred to as "callbacks" in other contexts.