Simplify reading LIGO_LW documents
This PR simplifies reading LIGO_LW-format XML documents by replacing the annoying ilwdchar_compat
system where documents are parsed twice, with a content handler-based system where ilwdchar
types are swapped with the relevant int
-like types on-the-fly automatically as documents are being parsed.
This allows to simplify everything else about reading LIGO_LW documents.