Improve organization of Dug import scripts

[gaurav]

We have an increasingly large number of import scripts in the bin/ directory. @YaphetKG suggested in a comment that we should move them into a new repository. If we were to do that, we could consolidate the other repositories containing import code in that repository, which would include:

• https://github.com/helxplatform/heal-data-munger
• https://github.com/helxplatform/heal-sparc-converter
• https://github.com/helxplatform/heal-phenx-kgx-convertor
• https://github.com/helxplatform/heal-sprint-converter
• https://github.com/helxplatform/kg_utils

I like the idea of having some basic importers in this repository (like the dbGaP one), but certainly more sophisticated ones like the HEAL Platform MDS import or HEAL VLMD import don't need to be in this repository.

This issue covers deciding whether we should create a single dug-importers directory for all these scripts, or whether they should be organized by project (e.g. heal-importers, bdc-importers, etc.).