XML ticket: metanorma/metanorma-model-iso#92

In order to reduce the number of flavour-specific tags in Metanorma, the following changes are proposed:

• Move annex/appendix to a Metanorma-wide tag
  • Outside ISO, IHO, BSI, appendix markup, [%appendix], is ignored, and an appendix to an annex is parsed like any other annex clause
• Replace executivesummary in NIST with clause[@type = 'executivesummary']
• Replace reviewernote in NIST with clause[@type = 'reviewernote']
• Replace submitters in OGC with clause[@type = 'submitters']