Only recompile when needed (or asked)

[jhoobergs]

If the exam.json file in the _output folder equals the new one that it would like to compile, no recompilation is needed.
When the user really wants, they should be able to recompile, by adding a no-cache options.

Except:

• When the numbas version changes: We can keep a file with the git hash in each exam output folder and compare that one
• When the version of one of the extension changes: We can keep a file with the git hashes of all used extensions in each exam output folder.
• When one of the resources changed. We can compare the resource files. (Things to keep in mind: https://stackoverflow.com/a/71799977)
• When the theme has changed: much harder to detect. Themes can also inherit from other themes, so we should also check whether any of the parent themes has changed (in files that were not overriden by the child). (https://docs.numbas.org.uk/en/latest/themes/index.html#themes)

Options for the theme:

• somehow hash the entire theme folder and compare it
• add a mandatory version file in the theme folder and just compared that one (and the once of the parents)
• compare the directory structures of the themes, if different, recompile, else compare each file byte by byte.