calculate_metrics: Calculate hydrologic metrics evaluated in the CSLS
In WDNR-Water-Use/CSLSscenarios: CSLS MODFLOW Scenario Analysis

This function calculates hydrologic metrics describing the magnitude, frequency, duration, rate of change, and timing of lake levels.

calculate_metrics(
  df_month,
  dts = c(1, 3, 12),
  metrics = c("median_level", "cv_level", "exceedance_level", "volume", "area",
    "mean_depth", "max_depth", "centrarchid_substrate", "vegetation_area",
    "exceedance_range", "depart_median", "median_dur", "cv_dur", "num_dur", "num_2yr",
    "median_rise_rate", "cv_rise_rate", "median_fall_rate", "cv_fall_rate", "fast_rise",
    "fast_fall", "good_spawning", "move_dock", "turtle_bay", "stratification", "is_lake",
    "paddlesports", "motorboat", "season_compare", "fast_rise_decade",
    "fast_fall_decade", "num_dur_decade", "num_2yr_decade",      "move_dock_decade"),
  dur_exceeds = NULL
)

`df_month`	data frame to use. Must include columns for "date" (must be POSIXct), "lake" (must be factor), and one with the lake levels (named in "col_name" argument).
`dts`	vector of dt values (months) to calculate metrics for, defaults to 1 (monthly), 3 (seasonal), and 12 (annual).
`metrics`	a list of which metrics to use. Defaults to all of them c("median_level", "cv_level", "exceedance_level", "exceedance_range", "median_dur", "cv_dur", "median_rise_rate", "cv_rise_rate", "median_fall_rate", "cv_fall_rate").
`dur_exceeds`	optional data frame with columns for "lake" and exceedance probabilities (e.g., "10", "25", "50", "75", "90) and associated lake levels. Used to calculate durations above/below these levels. Defaults to NULL to calculate durations on exceedance proababilities on provided time series.

summary, a data frame with the following columns:

`lake`	name of lake, character
`metric`	name of metric, corresponds with values in inputted "metrics" argument, character
`variable`	identifier for different types of metric. For example, the metric "median_rise_rate" will have variables of "1", "3", and "12" associated with it indicating the median rate over 1 month, 3 months, and 12 months, character
`value`	calculated value of the metric, numeric
`series`	type of time series used (monthly, seasonal, annual)