#656: mpl, utils, type_traits: check functionalities usage

[cz4rs]

fixes #656

Perform a basic cleanup to accompany checks and summaries for #656. This assumes we can rely on C++14.

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Eigen docs (we currently test with Eigen 3.4.0):
Eigen up to version 3.4 is standard C++03 and maintains reasonable compilation times. Versions following 3.4 will be C++14.

Trilinos docs:
https://docs.trilinos.org/files/TrilinosBuildReference.html#setting-the-c-language-standard-for-trilinos

[cz4rs]

Note: this might not be ready for merging yet, but removing is_same and enable_if_t should get us some CI feedback and demonstrate the scope of the changes required.

[fnrizzi]

@cz4rs how long do you think before this is done?

[cz4rs]

@cz4rs how long do you think before this is done?

@fnrizzi If we can make C++14 required for pressio, than this is ready for review. I will push header reorganization ("condensing") in a separate pull request.

Also: if we can bump the requirement to C++17, some additional improvements are possible (a simple one like using std::is_same_v should fit in here nicely).