Parameter for the NT database

[muffato]

Description of feature

In both #184 and #187, people had their blast database set up as 1 directory with many blast databases, not just NT.
The pipeline failed because it found multiple multiple .nal files.

I'm wondering if we should make the pipeline require the full path to the .nal file ? Things would be much more explicit and straightforward, and we may not need to run find at all.

That would be a breaking change, which we're "allowed" to do between 0.x and 1.0, but if we want to ensure backwards compatibility, then we could make the pipeline accept both a directory and a .nal.
If we keep the find, then we need to check how many .nal files it finds, and output a proper error message (maybe rewrite this find stuff in Groovy ?)

[yumisims]

same for this one