Issue #500: Reduce messages in bias_quantile()

[nikosbosse]

Fixes #500

Currently, bias_quantile() uses an internal helper function, bias_quantile_single_vector() to compute bias for a single forecast.

This helper functions throws a message if the median is not part of the quantile levels provided. That means that a single call to bias_quantile() can produce a lot of messages, as the message is repeated every time the helper is called.
This PR

• moves the message from the helper function to bias_quantile(), thus only getting one message for a single call to bias_quantile().
• adds a test for this

This is related to #418 (and only addresses a small part of the larger problem), but is in my opinion definitely an improvement over what we currently have.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b778f8a) 81.21% compared to head (5f182da) 81.22%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #501      +/-   ##
===========================================
+ Coverage    81.21%   81.22%   +0.01%     
===========================================
  Files           20       20              
  Lines         1725     1726       +1     
===========================================
+ Hits          1401     1402       +1     
  Misses         324      324              

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[seabbs]

Good idea. Looks good.