Refactor serial tbsv implementation details and tests

[yasahi-hpc]

This PR aims at improving the implementation and testing of serial tbsv.

• Moving implementation details into Impl namespace.
• Use OpID and OpConj for conj operations instead of do_conj variable
• Use Kokkos::is_view_v<AViewType> instead of Kokkos::is_view<AViewType>::value
• Covering all the unit-tests for all the combinations of Non-Trans/Trans/ConjTrans
• Remove using namespace KokkosBatched from the test code
• Rename variables starting from underscores in the test code

Added based on the discussions in #2497

• Add docstring and assertion for Arg template parameters

[lucbv]

I don't think the gesvd error is related to this so we could merge the PR, fine with you @yasahi-hpc ?

[yasahi-hpc]

I don't think the gesvd error is related to this so we could merge the PR, fine with you @yasahi-hpc ?

I agree gesvd error should be unrelated. Based on the discussions in #2497, I added docstrings and assertion about Arg parameters. If it is good for you, I think this PR is ready to be merged.

If we find errors in gesvd, I will open an issue and fix that.