add nblast code

[jefferis]

Should include routines both to fit score matrix and to search

Should it also include more generic versions of cached search functions?

And v1 nblast search (as used in frulhns paper)

[jefferis]

Might be better if this is in a separate package (e.g. when writing it up ...)

[jefferis]

i.e. nat.nblast (there might also be a nat.pipeline and/or nat.unu)

[jdmanton]

See https://github.com/jefferislab/nat.nblast.