Assertion macros for approximate equality

[milibopp]

I already added this to cgmath in rustgd/cgmath#131 and would like to contribute a similar feature here. The API differences are minor, so it should be mostly a matter of copy & paste. To make this self-consistent, I'll repeat the explanation:

Testing code with floating point numbers and nalgebra data structures one can use assertions on ApproxEq types:

assert!(approx_eq(&a, &b));
assert!(approx_eq_eps(&a, &b, &eps));

This does not yield readable error messages upon failure. Arguably it is also a bit ugly to read/write. I propose adding macros similar to assert_eq! to alleviate this, e.g.:

assert_approx_eq!(a, b);
assert_approx_eq_eps!(a, b, eps);

The question is, whether exported macros should go in a separate crate or not. Since macros aren't namespaced some libraries do that, to allow importing macros separately from the rest of the functionality, while others don't. I don't think that would make a lot of sense here though, as the macro is tightly coupled to the ApproxEq trait.

[sebcrozet]

In the long term, I wonder if this kind of assertion could be moved to https://github.com/bjz/algebra, formalized as a test of inclusion of a point in another point's neighborhood (i-e. for the case of Reals, an open ball of radius eps). Though this has more to do with topology than with algebra; so I am not sure this will happen.

I the mean time, I think this can be added to nalgebra.

[milibopp]

cc @bjz

[brendanzab]

+1 for this