Add weekly precipitation data

[dvictori]

I created a weekly precipitation data, trying to tackle issue #45

Not sure I did it right. Data comes from CHIRPS V2.0 and is aggregated to weekly using a 53 week year (lubridate::week()). From what I recall, it was not possible to use time series with fractional frequencies in SPI().

Here is an exemple on how to use the dataset
#' data(campinas)
#' campinas_ts <- ts(campinas$prcp, frequency = 53, start = c(1981, 1))
#' spi(campinas_ts, scale = 12)

Cheers and thanks for your great work

[sbegueria]

Thanks @dvictori, I'll review the DS and include it with an example.