Mathmaker

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* Patch version (include required files).

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* Patch version (fix pyproject.toml contents).

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* Make use of siunitx version 2, so far, when siunitx is the default version.
* New option --cotinga-template to create the templates to be used by cotinga.
* New directive: config, to print loaded configuration (loadable as json, if necessary)
* Minor improvements
* Maintenance.
* Do not compile all sheets created during tests to reduce the time spent.
* Rename all mental calculation sheets; organize them in 2 years; provide default titles
* Add --belts option to let the user provide its own belts names (use like this: --belts=/path/to/belts.json; belts.json must redefine some or all of the belts identifiers like in the default settings file mathmaker/settings/defaults/mc_belts.json)

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* New questions types for mental calculation: read abscissa of a point on an axis; find the missing length of a polygon, knowing its perimeter and other sides; calculate a simple fraction of a simple quantity. multi_direct questions may have 3 factors.
* Update yellow1, yellow2 and orange belts in mental calculation.
* Support python>=3.8 (drop 3.6 support)
* Several minor improvements.
* Bug fixes.

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* Bug fix.
* Minor modifications to some mental calculation sheets.

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* Add mental calculation questions: simple conversions between a fraction and a decimal; additions and subtractions of clock times.
* Bug fixes and upgrade of mental calculation sheets (white1 to yellow).