[Core] Proposition to reduce the number of alias declarations

[alxbilger]

The goal is to reduce the number of alias declarations. I propose 2 methods:

1. using traits (for example DataVecCoord_t<Out>). This is illustrated in Multi2Mapping.
2. using a set of macros. The aliases are still declared, but it is hidden inside the macro.

I find the trait approach elegant (nothing is hidden and no use of macro or additional code), but too much verbose when only one template parameter is available (e.g. TetrahedronFEMForceField).

The advantage of the macro is that it does not require changes in the code other than the alias declarations.

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[alxbilger]

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