CSLSlevels: CSLS Lake Level Analysis

Function contains Bob Smail's code for predicting lake levels using Bayseian Structural Time Series.

predict_lake_levels(
  study_lakes,
  lake_name = "Long Lake",
  start_date = "1938-01-01",
  seed = 2017
)

`study_lakes`	input data frame for BSTS calcs. Comes from "//dnr/programs/DG/DG_Projects/Water Use/WU_Central_Sands_Study/data_mgmt/hist_lake_levs/study_lakes_pred_levs_20200302.csv"
`lake_name`	name of lake to process. Defaults to "Long Lake".
`start_date`	first date to include in estimated lake level timeseries. Defaults to "1938-01-01".
`seed`	randomization seed for bsts. Defaults to 2017.