Simplify the testing types

[thoasm]

Add template-type functions to combine and merge type lists. As a result, the GINKGO_DPCPP_SINGLE_MODE only needs to be present once.

Additionally, change the typen name ValueAndIndexType to ComplexAndPODTypes (because gko::size_type is not an IndexType).

TODO:

• [x] Add documentation to the type helper
• [x] go through all existing tests and use the wrapper in all cases we define types on our own (example: CB-Gmres)