clear() didn't check if blocks was empty
cell.clearCache() called when cell not initialized
Force refresh of CellListFragment on resume with content of current group. When adding a new cell with a new group, clear CellCollection groupCache to force its recomputation. Always force refresh of CellGroupsFragment on resume. Fixes #5. Also, use "My Worksheets" for uncategorized cells.
use optString instead of getString in CellData ctor, output diagnostic if null but DONT throw. Fixes #16.
Merge branch 'master' of https://github.com/sagemath/android