Librdchebi

- Show Deuterium in depictions
- Calculate mass from molecular formulas even if mol has R groups

- apply ReapplyMolBlockWedging to get wiggly bonds in depictions

* auxilliary atom_is_r_group function

* change the way masses are calculated

* add isotopes to molecular formula

* added functions to deal with carbons aliased as R groups

* depict return None if molecule can't be parsed