Add alias for max order (#77)

* cli/gcdmodel.cpp: Fix typos in comments.

* cli/gcdmodel.cpp (CGcdData::fixModelAndSubmodelOrder): Fix typos in comments.

* cli: Support "max" as an alias to the maximum value for the order switch.

* cli/cmdline.cpp (parseArg): Understand "all" for the order switch.
(showUsage): Document it.
* cli/gcdmodel.cpp (CGcdData::fixModelAndSubmodelOrder): Ensure the
root model order gets recomputed when "/o:max" was provided as argument.
* cli/model.h (UNDEFINED_ORDER): Redefine as the minimum int value.
(MAXIMUM_ORDER): New constant.
* doc/pict.md: Document it.
* test/arg/.tests: Add test.

Co-authored-by: Maxim Cournoyer <maxim.cournoyer@savoirfairelinux.com>