[LinAlg] Prepare MultiVector class for Tpetra

[mayrmt]

Description

Sub issue from #875 to prepare the class Core::LinAlg::MultiVector for the switch to Tpetra.

Possible Solution and Definition of Done

• Do all necessary steps as outlined for Core::LinAlg::Vector in [LinAlg] Prepare vector class for Tpetra #1121
• Replace operator(int i) to access the ith vector in the MultiVector by the Tpetra-conforming get_vector(const int i). (Note: Tpetra internally uses size_t instead of int, but that is not the core issue here. Replacing the access operator with an access methods is at the core of this task). Reference to Teptra: https://docs.trilinos.org/dev/packages/tpetra/doc/html/classTpetra_1_1MultiVector.html#a7951dcc07f3da968cb2b9b9a4d5fdcdc (see Replace operator(int i) in Multivector with get vector function #1567)
• Remove MinValue, MaxValue and sum_into_local_value and ReciprocalMultiply since they are not available within Tpetra any more
• Develop strategy to handle FE_Vector. Do we introduce a second wrapper?
• Remove Implicit conversion from Interface to avoid missbehaviour
• Refactor Epetra_CombineMode and use Core::LinAlg::CombineMode instead (see Refactor CombineMode for MultiVector class #1589 )

[mayrmt]

@maxfirmbach, I assume that PR #1519 completed the open item "Refactor Epetra_CombineMode and use Core::LinAlg::CombineMode instead", right? Can we mark its checkbox as completed?

[maxfirmbach]

@mayrmt I forgot to adapt it in the multi vector. Maybe @dharinib98 can take care of it. It is a rather quick and simple cleanup task.

[maxfirmbach]

@dharinib98 Please make sure to keep this issue up-to-date if something has merged or new issues arise. Thanks!