Implementation of atropisomer recognition & encoding

[cm-beilstein]

In its current state the InChI doesn't recognize and encode atropisomers. Therefore, the current PR is about implementing routines that automatically identify the atoms and rotatable bonds that define the pivots of atropisomers. Furthermore, those atoms are then encoded in the InChI string according to task group https://iupac.org/project/2019-017-2-800/ .

Examples:
current InChI:
InChI=1S/C12H6Br2Cl2/c13-7-3-1-5-9(15)11(7)12-8(14)4-2-6-10(12)16/h1-6H
new InChI:
InChI=1S/C12H6Br2Cl2/c13-7-3-1-5-9(15)11(7)12-8(14)4-2-6-10(12)16/h1-6H/t11-/m0/s1 or
InChI=1S/C12H6Br2Cl2/c13-7-3-1-5-9(15)11(7)12-8(14)4-2-6-10(12)16/h1-6H/t11-,12-/m1/s1

• Add description
• Add examples with expected output
• Add molecule dataset (if anyone has good examples for testing, please forward them to me @cm-beilstein or any other member of the InChI developer team. Alternatively, they can be directly uploaded via the "Report"-button in the web demo (https://iupac-inchi.github.io/InChI-Web-Demo/). Thanks.)