fix: some documentation typos
docs: update formatting of XML schema excerpts
xmlch:  add baseline xmlchemy implementation
xmlch: add baseline pptx.oxml.simpletypes
oxml: convert coreprops to xmlchemy

First conversion, so includes transplant of xmlchemy and oxml base
classes.
xmlch: add RequiredAttribute
xmlch: add ZeroOrMore
oxml: convert pptx.parts.presentation to etree
oxml: convert CT_Presentation to xmlchemy
oxml: convert CT_SlideId to xmlchemy
oxml: convert CT_SlideIdList to xmlchemy
oxml: convert CT_SlideIdList to xmlchemy
oxml: convert CT_SlideMasterIdListEn.. to xmlchemy
oxml: convert CT_SlideSize to xmlchemy
xmlch: add OptionalAttribute
xmlch: add OneAndOnlyOne
xmlch: add OneOrMore
oxml: stable after objectify->etree conversion
rfctr: change all parse_xml_bytes -> parse_xml
oxml: add ST_Percentage