SPIspatial: Calculates SPI for every layer of raster brick
In Menkli/SPIspatial: Calculates SPI From Raster Brick

Calculates SPI for every layer of raster brick

1	SPIspatial(brick, timescale, start, frequency, ref.start, ref.end)

`brick`	Output brick from the build_brick function
`timescale`	From SPEI::spi. an integer, representing the time scale at which the SPEI / SPI will be computed.
`start`	When was the first measurement in the dataset? A vector with two numbers: e.g. c(1970, 1)
`frequency`	integer. If the timespan covered in the dataset are month, then use 12.
`ref.start`	When does the reference period start? A vector of two numbers: e.g. c (1980, 1)
`ref.end`	When does the reference period end? A vector of two numbers: e.g. c (2010, 1)