Enhancement: Throw EntityDefinitionAlreadyRegistered exception

[localheinz]

This PR

• throws an EntityDefinitionAlreadyRegistered exception when an entity definition has already been registered for a class name

Follows #86.

[codecov]

Codecov Report

Merging #106 into master will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #106      +/-   ##
============================================
+ Coverage     94.49%   94.54%   +0.05%     
- Complexity       77       80       +3     
============================================
  Files            10       11       +1     
  Lines           218      220       +2     
============================================
+ Hits            206      208       +2     
  Misses           12       12              

Impacted Files	Coverage Δ	Complexity Δ
...rc/Exception/EntityDefinitionAlreadyRegistered.php	100.00% <100.00%> (ø)	2.00 <2.00> (?)
src/FixtureFactory.php	91.80% <100.00%> (-0.07%)	45.00 <0.00> (+1.00)	⬇️

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