Detect magic in notebooks and issue warnings #17
Description
The current xcetool implementation for IPython magic commands (currently in progress in PR #16) is simply to turn them into no-ops. In many cases this is appropriate, for instance if they're set-up commands for inline display, which in any case won't be relevant in a generated compute engine or application package. However, xcetool should at lest detect magic commands anywhere in the input notebook and issue a warning to the user during conversion if they are present, since in some cases they could be important to the functionality.