Feature: central estimates off by default

[seabbs]

This PR:

• Adds a new plotting argument to all plot functions - central. This controls whether central estimates (i.e the mean and median) are shown. In a change to the default visualisations these are now not shown automatically but by setting central = TRUE the user can elect to add these estimates to the plot.
• Resolves low level check issues.

[codecov[bot]]

Codecov Report

Merging #108 (1cff03c) into main (5d5e627) will increase coverage by 0.12%. The diff coverage is 96.89%.

@@            Coverage Diff             @@
##             main     #108      +/-   ##
==========================================
+ Coverage   94.51%   94.63%   +0.12%     
==========================================
  Files          14       14              
  Lines         838      894      +56     
==========================================
+ Hits          792      846      +54     
- Misses         46       48       +2     

Impacted Files	Coverage Δ
R/forecast-tools.R	97.72% <85.71%> (-2.28%)	:arrow_down:
R/posterior-methods.R	92.98% <95.00%> (+2.78%)	:arrow_up:
R/model.R	98.41% <95.12%> (-1.59%)	:arrow_down:
R/forecast-evaluation.R	90.00% <100.00%> (+0.52%)	:arrow_up:
R/forecast-methods.R	80.00% <100.00%> (ø)
R/forecast.R	93.24% <100.00%> (+0.28%)	:arrow_up:
R/plot.R	93.03% <100.00%> (+0.83%)	:arrow_up:
R/postprocess.R	98.65% <100.00%> (+<0.01%)	:arrow_up:
R/utils.R	76.66% <100.00%> (+1.66%)	:arrow_up:

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