add read support for dta formats 120 and 121. closes #85

[JanMarvin]

Untested changes to support reading from dta formats 120 and 121.

The alias format is ignored when reading and a warning returned. Write support seems possible, but useless at the moment.

[JanMarvin]

This added support for dta format 120 and a .dtas file created from Stata 18. Handling for this file type is currently not implemented, but a workaround is implemented in the test. These so called frames contain multiple dta files in a zip file (the dtas file contains dta files), which can be linked together. In the example, the column income is merged from the second data file.

[JanMarvin]

Created a huge data file with 40,000 variables, numerics with labels. Worked smoothly.

[JanMarvin]

Support for 120|121 is atm experimental, not tested, but could be interested, if a 120 file is loaded and should be modified and stored again as 120 file.

[JanMarvin]

Tested the following for 120 and 121. This obviously isn't sufficient, but we have quite a few other tests.

library(readstata13)

n <- 100
k <- 4000 # or 40000 for 121: max for 120 is 32767
mm <- matrix(rnorm(n * k), ncol = k, nrow = n)

df <- as.data.frame(mm)
attr(df, "var.labels") <- paste0("var", seq_len(k))

save.dta13(data = df, "/tmp/huge_df.dta", version = 120)