Add CredibleInterval() as an internal function

[nfrerebeau]

The next release of ArchaeoPhases will introduce breaking changes (the CredibleInterval() function will be defunct).

CredibleInterval() is now an internal Baylum function and no longer needs to be imported from ArchaeoPhases.

[codecov]

Codecov Report

Merging #33 (6d7c8e1) into master (68aa1eb) will increase coverage by 0.17%.
The diff coverage is 68.75%.

❗ Current head 6d7c8e1 differs from pull request most recent head 4646f88. Consider uploading reports for the commit 4646f88 to get more accurate results

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
+ Coverage   68.39%   68.56%   +0.17%     
==========================================
  Files          18       18              
  Lines        2202     2214      +12     
==========================================
+ Hits         1506     1518      +12     
  Misses        696      696              

Impacted Files	Coverage Δ
R/Age_Computation.R	0.00% <0.00%> (ø)
R/Palaeodose_Computation.R	0.00% <0.00%> (ø)
R/AgeC14_Computation.R	87.50% <100.00%> (ø)
R/AgeS_Computation.R	81.84% <100.00%> (ø)
R/Age_OSLC14.R	62.68% <100.00%> (ø)
R/internals.R	100.00% <100.00%> (ø)

[RLumSK]

Thanks!