Make AbstractTracer subtype of Real

[adrhill]

Make AbstractTracer subtype of Real instead of Number. Closes #91.

Not sure whether we need to consider this as breaking, since tracer types are considered internals. However, the documentation mentioned tracers subtyping Number.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.91%. Comparing base (298837f) to head (92db98c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #92      +/-   ##
==========================================
- Coverage   78.92%   78.91%   -0.01%     
==========================================
  Files          17       17              
  Lines         707      702       -5     
==========================================
- Hits          558      554       -4     
+ Misses        149      148       -1     

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

[adrhill]

Not sure whether we need to consider this as breaking, since tracer types are considered internals.

This should be considered breaking because SCT might stop working on some functions it previously worked on.

[gdalle]

why did you add Complex everywhere?