process_levels: Process lake elevation, area, volume data for calcs
In WDNR-Water-Use/CSLSchem: CSLS Water Chemistry Analysis

Description Usage Arguments Details Value

Processes CSLS lake level data for water/solute balance calculations. Ensures no gaps in daily lake elevation data from CSLSdata::lake_levels, uses CSLSdata::elev_area_vol to convert daily elevations to areas and volumes, then summarizes to desired time step (daily or monthly).

1	process_levels(start_date, end_date, dt = "day")

`start_date`	start date of analysis period (POSIX).
`end_date`	end date of analysis period (POSIX).
`dt`	time step at which to summarize data. Defaults to "day" for daily time step, can also be "month" for monthly time step.

If using with non-CSLS data, will need an alternate way to get lake level, lake area, and lake volume data into the format outputted by this function.

df, a data frame with the following columns:

`lake`	name of lake, i.e., "Pleasant", "Long", and "Plainfield"
`date`	date of observation (POSIX). If monthly time step, monthly summary is assigned to first day of the month
`level_m`	mean lake elevation (mamsl) for the time step (daily or monthly)
`area_m2`	mean lake area (m2) for the time step (daily or monthly)
`vol_m3`	mean lake volume (m3) for the time step (daily or monthly)
`dV_m3`	change in lake volume (m3) for the time step (daily or monthly)