Prefix assume & assert definitions to prevent name clashes

[merwaaan]

Hi @juj

We use MathGeoLib alongside other libraries and recently suffered unexpected compilation errors whose cause took a bit of time to track. In the end, we found out it was actually due to MathGeoLib redefining assume, which broke other dependencies also using this common name (for instance, fmt: https://github.com/fmtlib/fmt/blob/739055ae7b08f8d5dffee9552334ede92e50b17b/include/fmt/format.h#L354)

This commit prefixes assume (as well as assert and mathassert for consistency) with "mgl_" to prevent that issue.
If you prefer, I could use another prefix.