[LinAlg] Refactor sparse operator, matrix and block-matrix wrapper

[maxfirmbach]

Description

Currently our LinAlg::SparseOperator inherits from Epetra_Operator, which brings a few difficulties with it. I would propose to remove this inheritance and provide just an abstract LinAlg::SparseOperator interface with necessary methods we need. The actual implementations, namely LinAlg::SparseMatrix and LinAlg::BlockSparseMatrix should internally hold the respective Epetra related object (similar to our other wrappers).

Possible Solution and Definition of Done

• Remove the inheritance to Epetra_Operator of LinAlg::SparseOperator (see [LinAlg] Remove the inheritance of Epetra_Operator from SparseOperator #1524).
• Remove the inhertiance to to Epetra_Operator of LinAlg::BlockSparseMatrix. The only (??) thing that prevents this is the epetra_operator() call in the iterative solver that needs the inheritance to be able to overwrite Apply() and ApplyInverse(). With [solver] Move internals to Thyra objects #1400 resolved, we should be able to cut the inheritance.
• Move member functions related to matrix addition to free functions (see [EpetraExt] Evaluate the sparse matrix addition methods #136)

Refactor LinAlg::SparseMatrix wrapper

• Think about how the wrapper could hold an Epetra_CrsMatrix as private member.
• Remove as many Epetra_CrsMatrix occurences in the code and replace by Core::LinAlg::SparseMatrix
• One more instance of get_epetra_export call remains: need our own Import implementation for Epetra_CrsMatrix. (see [Epetra] Cleanup some related calls #1107)
• Add a conversion utility function, which returns a Thyra::LinearOp wrapping the Epetra_CrsMatrix
• Turn on StaticProfile to true (see [LinAlg] Static sparsity layout for sparse matrices #1544)

Refactor LinAlg::BlockSparseMatrix implementation (see #1525)

• Remove the method Core::LinAlg::block_matrix2x2(), which is only used once in xfluid
• Remove the method Core::LinAlg::BlockSparseMatrixBase::get_partial_extractor(), which is not used
• Add a conversion utility function, which fills a Thyra::PhysicallyBlockedLinearOp and returns a Thyra::LinearOp

Interested Parties

@sebproell @knarfnitram @dharinib98