Support multiple version of the optional glam conversion

[sebcrozet]

This removes the convert-glam and convert-glam-unchecked features.
Instead, this adds the convert-glam013, convert-glam014, and convert-glam015 features for conversions targeting the versions 0.13, 0.14, and 0.15 of glam.

With this change of features naming, we can provide conversions for multiple versions of glam, and a major version bump for glam will no longer require a major version bump for nalgebra.

The conversions behind the convert-glam-unchecked feature have been replaced by TryFrom impls or conversions that transform the input to generate a valid output.