Small refactor of QBMM subroutines

[sbryngelson]

A small refactor of the QBMM subroutines.

[codecov]

Codecov Report

Attention: Patch coverage is 65.13761% with 38 lines in your changes missing coverage. Please review.

Project coverage is 43.42%. Comparing base (2f8eef1) to head (8ba6d18).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/simulation/m_qbmm.fpp	65.13%	30 Missing and 8 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #856      +/-   ##
==========================================
- Coverage   43.47%   43.42%   -0.05%     
==========================================
  Files          68       68              
  Lines       19766    19630     -136     
  Branches     2375     2353      -22     
==========================================
- Hits         8593     8525      -68     
+ Misses       9726     9676      -50     
+ Partials     1447     1429      -18     

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.