Increase test coverage

[cmdcolin]

It could be worth it to find test files to increase code coverage. Notably the LTF8 code is not completely covered for larger values, and these are somewhat tricky to encode

One source of data is https://github.com/samtools/hts-specs/tree/master/test/cram/codecs but they are not in a full cram files, just snippts of encoded data. they could be added as 'unit tests' of the codecs though