Move 'converters' to separate repository or repositories

[tgraham-antenna]

converters should be moved to a separate repository under the Schematron organisation. Since the code under converters does three different things -- or more than three, depending on how finely you want to divide ToSchematron -- it could be divided into multiple separate repositories.

Subversion lets you checkout part of a project -- e.g., just the converters part -- but Git doesn't. OTOH, Git, like Subversion, lets you have external submodules.

It would be cleaner if converters was in a separate repository that, if necessary, included the XSLT implementation as a submodule.

https://github.com/webb/schematron-impl, for example, is a fork of this repository where everything other than the XSLT implementation has been removed. This repository should end up like that.