Fix annoying warnings

Reference issue

What does this implement/fix?

This resolves all of the compiler warnings in clang default/avx2. There are many outstanding warnings in gcc. Summary of changes:

1. equal_strict no longer complains about comparing unsigned/signed integers. To address this, I check if the signed argument is negative, return false if so, otherwise cast it to its unsigned counterpart and do the comparison.
2. dot no longer complains about an unused typedef
3. pcarg no longer complains about an unused typedef
4. plainobjectbase::_init2 no longer complains about an unused typedef
5. ref.h enum comparisons
6. SVD uninitialized variables
7. arraycwise underflow issues with arm
8. deleted a bunch of stupid tests and split up the remaining tests in arraycwise to help with CI failures due to lack of resources
9. random_cast_without_overflow -- dont negate unsigned arguments
10. std_vector test doesnt complain about un-initialized memory (for a test that's not run!)
11. tensorblock: enum comparisons
12. FFT: change enum to constexpr variables to avoid shadowing existing Default enum in Core
13. disable deprecation warning in euler angles
14. NNLS no longer complains about unused typedef
15. tensor casts: use cast_impl instead of static_cast to correctly handle real->complex casts
16. tensor reduction: judiciously cast to double
17. make convert_index fancier by handling signed/unsigned integer range check

Additional information