Limits for saving dta-files

[JanMarvin]

While fixing the unsigned integers, I stumbled upon something else. There are potential problems with huge datafiles stored in older Stata versions, where we have to determine some limits. Stata has used different limits for SE and non SE dta-versions. We could end up in a case, where we ignore Statas limits and write a dta-file to big for Stata to read. Good thing is, it should not be a problem for us to reimport as long as the nrow(data) part is able to be stored in a uint32 since we do not have the same limits Stata has. Though right now, nothing permits a user to store a dataframe of length > uint32_t in non Stata14 dta-versions. Should be easy to add a limit check in 'save.R' and test limits with Stata (storing a "huge" file in older formats).