HELM ver 1 scope: support multiple sequences and connection tables (import and export)

[even1024]

Background
Indigo macromolecules support

Solution
Import and export of multiple RNA, PEPRIDE, CHEM in one string. Support connection tables.
Tests should include corresponding KET<>HELM<>MOL conversions.

HELM notation could be found here: https://www.pistoiaalliance.org/helm-notation/
Reference: HELM Specification 2_04-1.pdf
Decided to support only features that implemented in Ketcher already
That is

• 5.2 Simple Polymer section
  Implementation of two types of specific simple polymers: PEPTIDE, and RNA
  Implementation of non-specific simple polymer type: CHEM
  Implementation of unknown polymer type: BLOB
  Implementation of simple polymer notation (i.e. chains consists of polymers of supported types)
  Out of scope:
  Monomer Lists (pp. 5.2.4.1 should not be implemented)
  Monomer repeating units (pp. 5.2.4.2 should not be implemented)
  Monomer annotation (pp. 5.2.5 should not be implemented)
• 5.3 Complex Polymer
  Implementation of list of connection between backbone chains
  Out of scope:
  Unknown positions
  Hydrogen pairings (pp. 5.2.5 should not be implemented)
• 5.6 In-line HELM notation
  Implementation of SMILES and Extended SMILES formats support