Add unit tests for check_ functions and remove internal plots

[seabbs]

This PR increases test coverage by adding unit tests for check_ functions where missing. In adding these tests I noticed that check_quantiles was not as flexible as originally intended so I have refactored.

I have also removed two internal plot functions that were not be used and were untested.

[codecov]

Codecov Report

Merging #217 (5bdcfc1) into develop (93d3575) will increase coverage by 1.11%.
The diff coverage is 100.00%.

❗ Current head 5bdcfc1 differs from pull request most recent head ad75296. Consider uploading reports for the commit ad75296 to get more accurate results

@@             Coverage Diff             @@
##           develop     #217      +/-   ##
===========================================
+ Coverage    96.18%   97.29%   +1.11%     
===========================================
  Files           14       14              
  Lines         1521     1517       -4     
===========================================
+ Hits          1463     1476      +13     
+ Misses          58       41      -17     

Impacted Files	Coverage Δ
R/plot.R	100.00% <ø> (+8.33%)	⬆️
R/check.R	100.00% <100.00%> (+20.31%)	⬆️

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[adrian-lison]

All working as expected.

I added a separate error message for invalid probabilities in case we (indirectly) expose this more to users at a later point.