Convenient Vec methods

[JamesMcClung]

Mostly adds lots of mathematical methods to Vec:

• Reduce a single vector with sum, prod, mag, mag2
• Convert to an int vec with fint
• Vector products with dot and cross
• Elementwise and reductive min and max
• Invert with inv
• Iterate over N-dimensional indices with VecRange

Also adds an overload to rng::Uniform::get.

Most of these features aren't used in this PR. Future PRs will use them extensively, however.

[germasch]

👍 Thanks!

For the multi-d iteration, I guess I'll say that generally it'd be preferable to get rid of such loops altogether, and instead use gtensor expressions. But that's a bigger project, and may not work everywhere, anyway.

[JamesMcClung]

What gtensor expressions do you mean?