[NOX] Inherit matrix-free operators from SparseOperator

[vovannikov]

Description and Context

In order to proceed with removal of the base inheritances of NOX::Nln::Interface::RequiredBase and NOX::Nln::Interface::JacobianBase quite substantial modifications are required. This is just one of them and it effectively removes Epetra_Operator yet from one more place. It is also required for #1524.

A temporary wrapper NOX::Nln::MatrixFree::SparseOperatorWrapper is introduced here. Ultimately, it will be either simplified or completely removed later.

While working on this PR, I noticed that the default interface of Core::LinAlg::SparseOperator is excessive: most of the methods should be removed. For instance,

• is_dbc_applied() - it is very specific and is used in a single place of the code.
• add_other() - it is odd and not used at all.
  This is just what I identified by a quick look.

Related Issues and Pull Requests

#1524, #1535, #1486

[maxfirmbach]

@vovannikov I agree with the excessiveness of the SparseOperator interface. There are already a few issues to clean those things up e.g. #136 concerning the matrix addition methods and #863 as general effort to clean up our matrix implementations and interfaces.

[maxfirmbach]

Looks good to me.

[maxfirmbach]

I think we should aim to get rid of SparseOperatorWrapper again at a later point in time if possible.

[vovannikov]

I think we should aim to get rid of SparseOperatorWrapper again at a later point in time if possible.

@maxfirmbach, I fully agree. To prevent its unintended spread around the code, I made it a nested private class, such that it is impossible to use it in any other instance.