smc_pyutil/ipynb2sagews: fixing subtle bugs, adding support for "raw" cells, and refactoring cell conversion into separate class
smc_pyutil/ipynb2sagews: fixing subtle bugs, adding support for "raw" cells, and refactoring cell conversion into separate class

tscholl2/smc,DrXyzzy/smc,DrXyzzy/smc,DrXyzzy/smc,sagemathinc/smc,sagemathinc/smc,sagemathinc/smc,sagemathinc/smc,DrXyzzy/smc,tscholl2/smc,tscholl2/smc,tscholl2/smc,tscholl2/smc