docs: Have separate definitions for file, env. var, and URLs in SC/21.

Also correct or add missing usage of the three commands: \envvar for
enviroment variables, \file for filenames or parts thereof, and\url
for URLs.  At present they are all map to \texttt, but these
definitions allows other typography, if desired, to be used
consistently.
docs: Minor corrections and improvements up to Chapter 4 of SC/21.

It's mostly punctuation, grammar, and typography following a
re-reading of the manuscript.

Improvements include mention of showme to obtain SMURF and KAPPA
documents, and add MSB to the glossary.
docs: Adopt \model newcommand for SC/21.

Again it would be easy to change the appearance of all the model names
by altering one newcommand definition.
docs: Use \param command for parameters in SC/21.

This had only been partially implemented previously.
docs: There is no ERROR component in an NDF; call it an array in SC/21.
docs: Add STC-S definition and link to SC/21.
docs: Add an provshow example in SC/21.

Demonstratte the root ancestors option.
docs: Miscellaneous corrections in SC/21, mostly in Chapters 5 to 8.

It mostly corrects punctuation, typography and typing errors.  It also
clarifies the date in an example and use of xxx in a table of
configuration parameters, and how to access and use the contouring
facility in GAIA.
docs: Tidying in Appendix A.8's tables of SC/21.

Main thing is to use \param{} for parameter names in the name and
description columns.  Adjusted the widths of the columns slightly to
accommodate the wider tt fount.  Add "or not" to the descrptions
beginning "Whether...".
docs: Reword paragrapgh in SC/21 on the science processing at CADC.
docs: Miscellaneous fixes to Chapter 9 and appendices of SC/21.
docs: A few more minorr fixes to SC/21.

All but one were on my annotated copy, but were mised during editing.