Fix aerosol direct scaling

[chrisroadmap]

Previously, setting a scaling factor for a greenhouse gas species that also influences aerosol direct forcing also scaled that species' impact on aerosol direct forcing - this was not desired. This would only be an effect for CH4, N2O and HCs, on which the direct aerosol forcing impact is very small, which is why it was barely noticeable. Fixes #168

Please confirm that this pull request has done the following:

• Test results updated
• Description in CHANGELOG.rst added

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (bb2efd1) to head (387db6e).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #170   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           23        23           
  Lines          817       817           
  Branches       124       124           
=========================================
  Hits           817       817           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

---

🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests