Propbank file format conversion

[ijindal]

Current frame files are in html format. For consistency, convert them into xml format.

[arademaker]

we need to:

1. convert the HTML to XHTML
2. use XSLT to convert XHTML to the XML DTD from Propbank at https://github.com/propbank/propbank-frames/tree/main/dtds/v3.4