Added "fitted" function for flocker objects, and a bunch of smaller changes to improve functionality

[SimonCMills]

• added fitted_flocker() which generates expected values (or full distribution) of posterior & associated testthat file. It currently requires a bit of fudging of the flocker dataframe to pass new data, which this avoids, and doesn't require users to engage with the backend so much.
• Added a minimal test fit object so we don't have to fit a model each time we want to run unit tests.
• embedded fitted_flocker() within get_Z() to avoid code duplication and to allow users to easily pass new data that doesn't require the user to fudge in all the columns that flocker requires on the backend
• tweaked make_flocker_data() to give a significant speedup (particularly on large datasets where it is very slow currently).
• minor edit to make_example_data() allowing user to specify dimensions (n_species, n_pts, etc.)

[codecov-commenter]

Codecov Report

Merging #47 (1259ffa) into main (2660141) will increase coverage by 4.64%. The diff coverage is 58.75%.

@@            Coverage Diff             @@
##             main      #47      +/-   ##
==========================================
+ Coverage   21.19%   25.83%   +4.64%     
==========================================
  Files          10       11       +1     
  Lines         486      538      +52     
==========================================
+ Hits          103      139      +36     
- Misses        383      399      +16     

Impacted Files	Coverage Δ
R/example_flocker_data.R	100.00% <ø> (ø)
R/get_Z.R	0.00% <0.00%> (ø)
R/fitted_flocker.R	73.46% <73.46%> (ø)
R/make_flocker_data.R	68.51% <100.00%> (+0.89%)	:arrow_up:

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