Clean up HP/bucvac to allow file inclusions and ability for templates to fill Buc_* tags

[aelzenaar]

Clean up HP/bucvac to allow file inclusions and ability for templates to fill Buc_* tags

[aelzenaar]

So we need a New Bucvac Design:

1. Read TeX input, read metadata out of input
2. Read stencil, fill with jinjr, execute hooks to write metadata (e.g. Buc_title)
3. Output stenciled TeX to file and XeLaTeX it twice
4. Run add_record or update_record
5. Profit.

[aelzenaar]

Jinja allows us to add in custom filters, which means we should be able to write in a stencil {{ course ~ " assignment " ~ asstno | hp2_set_meta("Buc_title") }} instead of doing this manually. This is in line with the objective of replacing Pystache with Jinja outlined in the original issue comment.

With respect to allowing \input, the simplest solution seems to be exporting a tag like {{Hp2_stencildir}} so that something like \input{ {{Hp2_stencildir}}/myfunincludefile.tex can be written. Alternatively, we could use a filter like {{"myfunincludefile.tex"|Hp2_include}}, but this would be less desirable as it means that the user is forced to use \input instead of \include.