Add Cdd atom type to *3 in Intra_R_Add_Exocyclic reaction family.

This is a combination of several commits:

 * Add Cdd atom type to *3 in Intra-R-Add-Enxocyclic reaction family.
 * Added some comments to the longDesc fields.
 * Remove .DS_Store that shouldn't be there
 * Fix minor syntax problems caused by space, also group definitions are case sensitive(Cdd)!
 * Fix typo in Intra_R_Add_Exocyclic

Authors:   Fariba Seyedzadeh Khanshan <seyedzadehkhanshan.f@husky.neu.edu>
           Richard West <r.west@neu.edu>
Allow *3 to be Cdd in the DoubleBond group as well in Intra_R_Add_Exocyclic

The top level groups all allow Cdd atom type as *3, but we would not
descend the tree to get any decent data, because the double bond group was
defined as Cd only. Now at least we will get to a level two node, although
we have no relevant data below this.
Strip whitespace at end of lines