Better handling of cells that contain string 'nbdev_export('

[ozpau]

Currently, if you just happen to include string nbdev_export( in your code cell, the whole cell will be silently ignored by nbdev_prepare, nbdev_test, nbdev_proc_nbs and nbdev_preview. If this cell happens to contain code that is exported or rendered in docs, it would result in the corresponding command failing with hard to decipher error.

I have removed the corresponding check from the nbdev code. I am not sure we need it, but I can make a version of this pull request that handles that more gracefully by injecting monkey patching code into notebooks, something like this:

k.run_cell('import nbdev\ndef noop(*args, **kw): pass\nnbdev.nbdev_export = noop')

I imagine the check was there to prevent infinite loops of nbdev_prepare calling itself, but this doesn't happen because the check is for nbdev_export and not for nbdev_prepare.