Closes #149; store DNA sequence in domains and loopouts, not in strand

[UnHumbleBen]

Description

Main changes to review:

• Replace Strand.dna_sequence field with Strand.dna_sequence() and replace Domain.dna_sequence() with Domain.dna_sequence
• Test cases assertions rewritten so that domains store expected sequence
• Reimplemented functions
  • Strand.insert_domain
  • Strand.remove_domain
  • Strand.ligate

Related Issue

149

How Has This Been Tested?

Ran unit tests.

Screenshots (if appropriate):