Fix printing Manifold and Differential in diffgeom package

    The blackboard bold font in the AMSFonts package only has capital
    letters.  So, if we want to print small leters (e.g. "d" for
    the differential operator symbol), we shouldn't use
    \mathbb{d} (or "d" just disappear).
Merge pull request #1935 from skirpichev/diffgeom-printing

Fix printing Manifold and Differential in diffgeom module