refactor: rename solutions to transitions

[redeboer]

The Result class by now (#459) is just a container with a solutions list and a formalism_type. In the long term, the idea is that generate_transitions only returns transitions as a StateTransitionCollection object (see #458) and that a formalism_type does not have to be carried around within the reaction module (#168).

In preparation to this, the variable names are shifting from graph/solution to transition.

[codecov]

Codecov Report

Merging #481 (2f9d4a1) into master (0f1e5bd) will not change coverage.
The diff coverage is 90.90%.

@@           Coverage Diff           @@
##           master     #481   +/-   ##
=======================================
  Coverage   90.04%   90.04%           
=======================================
  Files          22       22           
  Lines        3296     3296           
  Branches      830      830           
=======================================
  Hits         2968     2968           
  Misses        170      170           
  Partials      158      158           

Flag	Coverage Δ
unittests	90.04% <90.90%> (ø)

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Impacted Files	Coverage Δ
src/expertsystem/reaction/__init__.py	86.53% <88.88%> (ø)
src/expertsystem/amplitude/helicity_decay.py	90.34% <100.00%> (ø)