Texext

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Configuration change:

* Changed method of pointing mathcode directive at the context for plots. The
old method attached a module to the sphinx config, making the config
impossible to pickle, so forcing a rebuild of all pages for each call to
sphinx-build. The new method uses a string that we use to import the
attribute containing the plot context.

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Bugfix release:

* Fixed processing of backslash escape characters in ReST input with dollars.

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Bugfix release:

* Use different method to get text to parse in Sphinx pages. We were
previously getting the raw ReST text and trying to do partial parsing with
regexps, but this was getting brittle, and there were already some nasty
bugs to do with the order we were putting back stuff we had taken out.
Instead, look for math dollars in the parsed doctree, so we can avoid
things like literal blocks via the ReST parser. We continue to use the
regex parser on docstrings though;
* Deal with multiple backticks used as sphinx section underlines, confusing
the regex parser;
* No longer exclude all indented lines from math dollar parsing, because it's
fairly common to have math dollars in list items, which are indented;
* Extend tests to cover math in list elements;

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Bugfix release:

* Fix use of custom plot_directive;
* Some testing fixes;

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Initial release:

* Refactored ``math_dollar`` extension to exclude more potential false
positives, add tests;
* Add ``mathcode`` directive.