Conservative Temperature of Freezing Seawater

## Implementation Note

This R function uses a wrapper to a C function contained within the GSW-C system as updated 2022-10-11 at https://github.com/TEOS-10/GSW-C with git commit `657216dd4f5ea079b5f0e021a4163e2d26893371`.

The C function uses data from the `library/gsw_data_v3_0.mat`

file provided in the GSW-Matlab source code, version 3.06-11.
Unfortunately, this version of the mat file is no longer displayed on the
TEOS-10.org website. Therefore, in the interests of making GSW-R be
self-contained, a copy was downloaded from
http://www.teos-10.org/software/gsw_matlab_v3_06_11.zip on 2022-05-25,
the .mat file was stored in the developer/create_data directory of
https://github.com/TEOS-10/GSW-R, and then the dataset used in GSW-R
was created based on that .mat file.

Please consult http://www.teos-10.org to learn more about the various TEOS-10 software systems.

## Examples

```
SA <- c(34.7118, 34.8915, 35.0256, 34.8472, 34.7366, 34.7324)
p <- c( 10, 50, 125, 250, 600, 1000)
saturation_fraction <- 1
CT <- gsw_CT_freezing(SA, p, saturation_fraction)
stopifnot(all.equal(CT, c(-1.899683776424096, -1.940791867869104, -2.006240664432488,
-2.092357761318778, -2.359300831770506, -2.677162675412748)))
```