Deprecate PicardEigen in favor of Eigenvalue #646
Description
Reason
PicardEigen is an old executioner in Griffin derived from NonlinearEigen with fixed-point iteration capabilities added to it. It's capability is reproducible with the newer Eigenvalue executioner and therefore it should be deprecated.
Design
Change PicardEigen to Eigenvalue in inputs and documentations.
Impact
Inputs and documentations use the latest recommended options.