Possible interface breaking in Ginkgo 2.0.0

[yhmtsai]

Track the possible interface breaking we find now.

1. csr strategy (Improving Csr::strategy_type #320 )
2. csr clac_size -> calc_size
3. Rename / Rebrand / Add executor information (e.g. change warp_size so something neutral, like simd_units)
4. Change SellP slice indexing from size_type to int64 (Inconsistent use of integer types #430)
5. Improve the preconditioner::Ilu interface (Improve ILU preconditioner interface #472)
6. Adding a Tag or similar to a LinOp, for example to know it is a lower or upper triangular matrix
7. Make Transposable, Permutable, ... return concrete types as well as LinOp
8. Improve Permutable/Permutation interface: apply + rapply, find alternative for Array?
9. Let device executors only take OmpExecutor parameters, not plain Executor
10. Adding a memory space abstraction (Add a memory space abstraction #654 )
    a. Logger needs to have a memory_space parameter, which breaks interface. Cannot use memory_space getter from Executor due to incomplete type issues.
11. Rename lt_strategy -> lh_strategy in ParICT parameters
12. Move make_temporary_clone to internal headers
13. Update loggers with implicit_sq_resnorm.
14. Remove ValueType from Identity
15. Split Csr IndexType into ColIndexType and RowPtrIndexType (64 bit column indices are rarely necessary)
16. Remove redundant information from Sellp (slice_lengths can be computed from slice_sets)
17. Remove Diagonal::read
18. Improve Hybrid strategy
19. Remove Array default constructor
20. Rename Array -> array
21. Replace Dense::at by either a device_accessor that makes clear that it can't be used on the host or device_reference which implicitly copies.

Topics to discuss:

1. Rename LinOp to Op or Operator
2. Change default behavior in Dense from multivector to dense matrix (Clarify the behavioral differences between a dense matrix and a multivector #796)
3. Rename Dense::create_submatrix to Dense::create_submatrix_view.
4. Remove the OmpExector inheritance from ReferenceExecutor.

If there is anything miss, please feel free to edit it.