Include intercept in get_coeffs calls

[tsalo]

Closes none. I was just thinking that we should include an intercept term when calculating echo-wise betas.

Changes proposed in this pull request:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.79%. Comparing base (af5e99a) to head (04278f1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1089      +/-   ##
==========================================
- Coverage   89.80%   89.79%   -0.01%     
==========================================
  Files          26       26              
  Lines        3540     3537       -3     
  Branches      621      621              
==========================================
- Hits         3179     3176       -3     
  Misses        212      212              
  Partials      149      149              

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

[tsalo]

Weirdly, including the intercept didn't change anything. I'm not sure why. All of the other components should have mean values of zero, right? So how could raw echo-wise time series (i.e., with different scales) have the same OLS fit with and without an intercept term?