Updates for SAS length

[elimillera]

This closes #20.

I've added a SASlength function to set and return the SASlength attribute similar to the other attribute setters. I've updated the documentation, NAMESPACE, and NEWS.md files as well.

Let me know if there are any changes you want to see before sending to CRAN. R CMD CHECK is all green.

> library(SASxport)
> mtcars2 <- mtcars
> mtcars2$mpg <- as.character(mtcars2$mpg)
> SASlength(mtcars2$mpg) <- 20
> mtcars2$am <- as.character(mtcars2$am)
> write.xport(mtcars2, file = "test.xpt")

[warnes]

Add SASlengrh examples to the two man pages and see the other comments.

[elimillera]

@warnes Thanks for the quick review. Updates have been made.

[elimillera]

@warnes Any other changes you'd like to see on this?

[warnes]

Code looks good! Just a couple of minor nits.

I'm going to pull down the code and run some checks now.

-G

[mstackhouse]

@warnes Any chance this could be merged soon? The length update here would allow CDISC compliant XPT file generation, which is going to be a huge benefit to the pharma industry's use of R.

[elimillera]

@warnes Do you have a time frame for this being pushed up to CRAN?

[DennisSweitzer]

Good to see these updates; My team just duplicated this before seeing you fixed it.
We also found another limitation. SAS allows integers to be defined with <8 bytes of memory, but SASxport converts all integers into an 8 byte double before putting it in the buffer.
Are there any plans to fix that? I found the code in xport.numeric() that needs modification, but haven't worked with C libraries before.

[mstackhouse]

@warnes do you intend to move this version to CRAN anytime soon? It would be immensely valuable to have this PR incorporated and available to install via CRAN.