Add a Convergence object that combines others

[joshuahansel]

Motivation

A desired convergence criteria can be composed of many elements. For example, a user may want to check a residual criteria and a variable step criteria.

Design

Have an object called CombinationConvergence that could do something like this:

[Convergence]
  # define conv1, conv2, conv3 above
  [conv4]
    type = CombinationConvergence
    converged_expression = 'conv1 || (conv2 && conv3)'
  []
[]

which can combine other Convergence objects using and/or statements and parentheses, at a minimum.

Note: if any return "diverged", then this would diverge. Alternatively, this could just be the default behavior and allow the user to specify:

diverged_expression = 'conv1 || conv2'

If we wanted a little more convenience/flexibility, we could ParsedConvergence instead, which would additionally allow boolean expressions instead of just boolean values:

converged_expression = 'conv1 || (ppA < tol)'

Impact

New object.