type security to enumerated string, extra test of non-class enum

[rboston628]

Description of work

Added a check to EnumeratedString to ensure class E is an enum. More tests of comparisons to underlying type of the enum.

Purpose of work

Ahead of lightning talk on EnumeratedString at mantid dev workshop 2023, I revisited this and added minor refactors.

Summary of work
Adding the line

static_assert(std::is_enum_v<E>);

which causes a compiler error if attempting to make an enumerated string without using an enum as the class.

Added some addition tests that verify behavior if not using an enum class, that the EnumeratedString can be compared both with the enum and with the underlying type.

Further detail of work

That's all there is.

To test:

Within EnumeratedStringTest.h, try adding the following to the namespace at L20:

struct NotAnEnum { static const int enum_count = 1; };
const std::vector<std::string> notAnEnum {"1"};

and inside the test class add:

  void testNotAnEnum() {
    EnumeratedString<NotAnEnum, &notAnEnum> bad (1);
  }

and try to compile. This should generate an error like the following:

mantid/Framework/Kernel/inc/MantidKernel/EnumeratedString.h:43:22: error: static assertion failed
      |   static_assert(std::is_enum_v<E>); // cause a compiler error if E is not an enum
      |                 ~~~~~^~~~~~~~~~~~
In file included from mantid/build/Framework/Kernel/test/EnumeratedStringTest.cpp:18:

There is no associated issue.

This does not require release notes because it is refacroting an internal feature that is not user-facing.

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[rboston628]

I locally ran

cmake -DDCS_SCREENSHOTS=on .
ninja docs-html

just to be sure.

[bolotovskyr]

This PR adds an important type safety check which should steer developers towards the correct usage of EnumeratedString. Also, a new unit test with a non-class enum is very useful. I have built the branch locally and verified that the compilation fails if we try to instantiate an EnumeratedString object with a non-enum type.