Allow user to specify the level of the posterior credible intervals for the phenometrics. (#15)

* test commit

* update README.md

* add credible interval spec

* add cred int spec

* add cred int spec

* update fit cred int for PlotBSLP

* finalize cred_int_level feature

Author: mattshisler

DESCRIPTION

@@ -13,7 +13,7 @@ License: MIT + file LICENSE
 Encoding: UTF-8
 LazyData: true
 Roxygen: list(markdown = TRUE)
-RoxygenNote: 7.2.1
+RoxygenNote: 7.2.3
 Imports:
     coda,
     data.table,