Handling alternative versifications

There are two aspects to handling alternative versifications. Firstly alternatives are just that, so we should be able to ignore them and use the base versification. So it would help to have a switch to turn them off. On the other hand it's not too hard to write:

'\\va .*?\\va\*' > ''
'\\ca .*?\\ca\*' > ''

in a changes file.

Secondly, \ca only gets used in conjunction with a \c. So we need to handle the case where there is a chapter split in one versification but not in another. For example in PSA the western protestant tradition has chapter 9 then 10, but other traditions join those psalms, and then split psalm 146. In both cases then \ca calls for a preceding \c. So you might have:

\c 9
...
\c 10 \ca 9\c*
\q1 \v 1 \va 22\va* ...

If we are following the alternative versification scheme, then we don't want a chapter break at that point. Why not? because the text of the chapter identifier has not changed, it is still 9. So this is a proposal to say:

If the presented chapter identification text does not change after processing \c, then ignore the \c completely. Notice this is based on processing \ca and \cp before making that decision.

No files, tests, or entry points are named. Start by tracing the Python/XeTeX handling of \c, \ca, \cp, and alternative versification markers, using the Psalm examples in the issue; done means alternatives can be disabled and unchanged processed chapter identifiers do not create a chapter break.