De-jankify implementation of DOP in polymer building

[timbernat]

The current way DOP (degree of polymerization) is defined in polymers.building.build_linear_polymer() is highly non-intuitive and case-specific to an AB/BA sequenced linear polymer. Planned Improvements to this include:

• Removing the DOP - 2 hardcode
• Replace DOP input with num_monomers input (which is much less ambiguous)
  • Correcting internals with explicit calculation based on requested DOP, sequence length, and the two terminal groups, namely DOP_mbuild = (num_monomers - 2) / len(sequence), with failure when this is not an int (incompatible sequence)
• Correct logging of target DOP to reflect these changes
• Update polymerist_examples to reflect these changes