Refactor ecm_prep.main()

[aspeake]

Refactors the main() function in ecm_prep.py. Summary of changes includes:

• Move ECMPrepHelper class into new module to reduce ecm_prep.py file size
• Extend ECMPrepHelper with new methods called from main():
  • get_measure_savings_shapes() import ecm prep shapes if applicable
  • get_existing_measure_summary_data() import previously prepared measure attributes
  • get_pkg_env_sep_info(): import envelope isolation measures, if applicable
  • get_indiv_measures(): collect individual measures that require updates
  • get_measure_packages: collect measure packages that require updates