Fix -Wuseless-cast compiler warnings

[graebm]

Issue #:
#973

This warning exists in GCC only (not Clang), and for C++ only (not C)

Description of changes:

• Add -Wuseless-cast warning to the "header checker"
• Remove useless casts in array_list.inl
• Ignore -Wuseless-cast warning in math.inl
  • The tricky part is that SOME platforms (Apple and BSD variants) define size_t as unsigned long and require casting to uint64_t (aka unsigned long long). While other platforms define size_t as unsigned long long and warn about useless casts if you cast it to uint64_t. My first commit attempted to only do casts on those platforms, but it was more complex than simply ignoring the warning.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.