Initial XSLT for producing OSCAL catalogs in HTML and PDF

[wendellpiez]

Addressing #4, this PR migrates an XSLT code base for publishing (formatting) OSCAL, first developed in an OSCAL repo development branch.

With respect to catalogs and profiles, this addresses usnistgov/OSCAL#574, Also, the PR updates the XSLTs to current models (so also usnistgov/OSCAL#630).

"NIST SP emulation" looks pretty good for SP800-53 catalogs, both Rev 4 and Rev 5, in both HTML and PDF. The "generic preview" stylesheet is comparatively rough, but beats looking at tags.

Remaining work on this task:

• Documentation, with potentially a little development of CLI or other tooling to help users run Saxon and Apache FOP.
• This includes documenting capabilities, limitations, dependencies, and how to modify/extend (with XSLT or only CSS)
• There is much more to be done on the SSP and other source formats

[david-waltermire]

Can you add a README.md to the XSLT directory to provide a quick overview of what can be found in the subdirectories? A one or two sentence bulleted list should be fine.