Force numeric for prediction and true value when scoring

[seabbs]

This PR closes #198 by forcing the conversion of true_value and prediction as indicated by @nikosbosse (the author of scoringutils) as the likely fix.

It also increments the dev version and adds a news item (the review might consider adding a reviewed by tag to this once their review is complete).

The following code now works as expected and package tests are passing.

library(epinowcast)
 
     nowcast <- enw_example("nowcast")
     summarised_nowcast <- summary(nowcast)
     obs <- enw_example("observations")
 
    enw_score_nowcast(summarised_nowcast, obs)

[codecov-commenter]

Codecov Report

Merging #201 (599f80b) into develop (edb2254) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 599f80b differs from pull request most recent head e400f8a. Consider uploading reports for the commit e400f8a to get more accurate results

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@@             Coverage Diff             @@
##           develop     #201      +/-   ##
===========================================
+ Coverage    88.18%   88.20%   +0.01%     
===========================================
  Files           14       14              
  Lines         1490     1492       +2     
===========================================
+ Hits          1314     1316       +2     
  Misses         176      176              

Impacted Files	Coverage Δ
R/model-validation.R	95.45% <100.00%> (+0.45%)	⬆️

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[seabbs]

LGTM (self-review) building on @pearsonca review whose suggestions were adopted without any additional changes.