modify nist loader to support multi-line GCM sections

The current NIST loader code assuems each section is unique. In the GCM vectors each set of section headers is unique, but the taglen (which, as the last section header, is what the current loader uses) is not. This PR changes the loader to concatenate the section headers for GCM such that they're guaranteed unique (example below)

```
[Keylen = 128]
[IVlen = 96]
[PTlen = 256]
[AADlen = 160]
[Taglen = 120]
```