Allow user to specify the level of the posterior credible intervals for the phenometrics.

[mattshisler]

• Added the cred_int_level parameter to the FitBLSP and FitBLSP_Spring functions with a default of 0.9 (90% credible interval)
• Included an explanation in the function documentation and re-generated the documentation files using roxygen2.
• Added a check to make sure the cred_int_value is between 0 and 1, exclusive.
• Updated the BlspFit class to include the credible interval level used.
• Updated the vis_fit function to label the produced plot with the credible interval level identified from the BlspFit object.
• Updated vignette so that the changes are consistent with the displayed graphs and test data. I did not add any content explaining the new feature, however.

[mattshisler]

Sorry, it looks like the entire vignette markdown is treated as "different" which is driving the 1,000+ deleted/added lines.

In reality I only updated the vignette markdown to reflect correctly labeled credible intervals in the plots.

[mattshisler]

Looks like it "diff"ed the entire mod_fit files as well.