calculate_durations: Calculate durations
In WDNR-Water-Use/CSLSlevels: CSLS Lake Level Analysis

Given a data frame with a "lake", "date", and "level" columns as well as a vector of desired exceedance probabilities, calculates consecutive months at/above (for probabilities <= 50%) or at/below (for probabilities > 50%) the exceedance levels.

1	calculate_durations(df, probs = c(10, 25, 50, 75, 90), departures = FALSE)

`df`	a data frame with a "lake" and a "level" column
`probs`	a vector with all exceedance probabilities to calculate. Defaults to c(10, 25, 75, 90).
`departures`	logical defaults to FALSE. If TRUE, calculates durations 1ft above median and 1ft below median

durations, a data frame with the following columns:

`lake`	name of lake
`variable`	exceedance probability, e.g., "10", "25", "75", or "90"
`value`	one count of number of months levels were consecutively above or below the given exceedance probability