Fix PCSG handling

[THinnerichs]

• add make_probabilistic
• normalize! not exported + not defined for non-probabilistic case #102
• Grammar printing became ugly? #103
• add prettier Base.show for probabilistic grammars.

[codecov]

Codecov Report

Attention: Patch coverage is 82.50000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 61.25%. Comparing base (beb1b25) to head (3f546a5).
Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
src/csg/probabilistic_csg.jl	75.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #104      +/-   ##
==========================================
+ Coverage   56.45%   61.25%   +4.79%     
==========================================
  Files           7        7              
  Lines         542      542              
==========================================
+ Hits          306      332      +26     
+ Misses        236      210      -26     

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[ReubenJ]

Not sure on #102, because other methods currently display a warning and assume uniform probabilities.

If you want to add an extra Base.show method for probabilistic grammars, we can either define this in Core or define it over CSGs instead of AbstractGrammars.

[THinnerichs]

Will discard prettier Base.show for now.

[ReubenJ]

With the tests for non-probabilistic cases, this looks good to me 👍