calc_bioclim_raster_old: Bioclimatic index calculation
In sitscholl/rebecka_package: Utility functions used in the REBECKA project

This function allows you to calculate bioclimatic indices based on the overlay of raster datasets.

1	calc_bioclim_raster_old(tx, tn, index.calc, frml, agg.func, period, set.zero)

`tx`	Raster brick with daily rasters of maximum temperature. The names of the rasters have to be the same as the names in 'tn' and must correspond to the day which the values refer to.
`tn`	Raser brick with daily rasters of minimum temperature. The names of the rasters have to be the same as the names in 'tx' and must correspond to the day which the values refer to.
`index.calc`	Name of the index that you want to calculate. There are some predefined indices within this function (Currently: Winkler, Huglin, GSTavg). If your index is not within this list you have to supply the necessary parameters manually and can leave out this parameter.
`frml`	A formula object that describes how to calculate the daily index values. The left side of the formula will be used to name the resulting RasterLayer and the right side to calculate the daily rasters.
`agg.func`	A function that will be used to aggregate the daily rasters calculated by 'frml' for every year
`period`	A sequence of numbers that indicates the day of years that should be used to aggregate the index
`set.zero`	Are values below 0 possible or should they be converted to 0?

calc_bioclim_raster(tx = tmax_brick,
  tn = tmin_brick,
  frml = winkler ~ ((T_max + T_min) / 2) - 10,
  agg.func = sum,
  period = 91:304,
  set.zero = T)