Parse triple-indexed objects

[miaoyinb]

Reason

Triple indexed arrays have been found necessary to keep a clear input syntax for some classes such as #21854.

Design

First it will require update in libMesh to enable 3D vector as print_helper()'s argument. libMesh/libmesh#3380

Then, Parser.C needs to be updated to cover triple-indexed arrays. (similar to #6459)

Impact

Clearer input syntax.

[YaqiWang]

In extreme cases, I suggest to have your own parsing function inside the object. The parameter can be in type of a plain string.

[miaoyinb]

In extreme cases, I suggest to have your own parsing function inside the object. The parameter can be in type of a plain string.

A triple indexed array is still not that complex. But I agree that your suggestion could be a good approach for future extreme scenarios.