Fix spline interpolation between different unit physical types

[rosteen]

Fixes #1186. Part of this accidentally snuck into a commit on main while I was doing the last release 😬 (changing from index 0 to using min and max) but this completes the fix. @StuartLittlefair would you confirm that you get what you expected in #1186 with this fix?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.95%. Comparing base (c4b90ed) to head (f275690).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1190   +/-   ##
=======================================
  Coverage   86.95%   86.95%           
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  Files          63       63           
  Lines        4568     4568           
=======================================
  Hits         3972     3972           
  Misses        596      596           

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[StuartLittlefair]

Thanks @rosteen - this is perfect