make exportprismia remove code cell tags

[brownsarahm]

code cell tags have syntas like:

:tags: ["skip-execution"]

these lines should be removed because prismia does not parse them right

[brownsarahm]

swap lesson class to use nbconvert or similar so that it can rely on parsing tools already written for the notebook format

[brownsarahm]

removing can be done within the wrap function or right before that.