summarise_isotopes: Monthly Isotope Measurements
In cvoter/isoH2Obudget: Lake Water Budget Using Stable Isotopes

Summarizes isotope measurements at a monthly timestep for a single lake

summarise_isotopes(
  isotopes,
  dictionary,
  timeseries,
  lake_levels,
  gw_levels,
  threshold = 0.01
)

`isotopes`	a data frame with isotopes measurements as formatted in the isotopes dataset, subset for a single lake.
`dictionary`	a data frame with site id numbers and static isotope site classifications as formatted in the dictionary dataset, subset to records for the lake of interest.
`timeseries`	a list with dates (POSIXct) to analyze, and intervals (lubridate interval) covering the month before each analysis date.
`lake_levels`	a data frame with daily water level measurements as formatted in the lake_levels dataset, subset to lake level records for the lake of interest.
`gw_levels`	a data frame with daily water level measurements as formatted in the lake_levels dataset, subset to groundwater level records at the lake of interest.
`threshold`	minimum median difference between lake levels and groundwater levels during the month of measurement in order to classify groundwater measurement.

monthly_isotopes, a data frame with the following columns:

date: first of the month for each monthly observation
d18O_pcpn: mean precipitation stable isotope measurement for the month
d18O_lake: mean lake stable isotope measurement for the month
d18O_GWin: mean groundwater inflow stable isotope measurement for the month
d18O_GWout: mean groundwater outflow stable isotope measurement for the month
GWin_sites: well sites used in mean d18O_GWin each month
GWout_sites: well sites used in mean d18O_GWout each month