Currently this is not implemented for initial ranking as it
is currently not accessible. (Defaults to NaN.)

Some parts are currently implemented in a performance wise
suboptimal way.

Tests now use the test_ prefix and are not in
sub-modules.

This includes a test of the `compute_analytics` methods,
and a port of the test from the prev. dart implementation
(as far as applicable).

- skip if no history available for a document
- if we have no relevant historic information at all
  we do return an error
- use fold in dcg
- in pick highest skip search if we won't be able to insert
  it anyway
- use Iterator::fuse() instead of FusedIterator

The new function is called nan_safe_f32_cmp.

assert!(approx_eq!(f32, left, right, ulps=n)) has the
problem that it doesn't print information about the
values when it fails so assert_f32_eq adds a format
string which on failure will be used as error message.