clang-tidy: Enable bugprone-integer-division

clang-tidy: Enable bugprone-integer-division

- Using int{} constructor syntax for intentional
int division with truncation in fp context.
- Adds new exactDiv which does integer division
and asserts that division is without remainder.
- Converts remaining divsion to fp division.

Also make GMX_ASSERT work in constexpr fn.
Convert over_alloc_* from macro to function.

Change-Id: Ieaeedd4558865a9dd541d24cf246f5c5733e87a1