Types for rational maps

[HechtiDerLachs]

CC: @wdecker , @jankoboehm

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 32 lines in your changes are missing coverage. Please review.

Project coverage is 83.25%. Comparing base (0e79592) to head (a281b89). Report is 54 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3683      +/-   ##
==========================================
+ Coverage   82.70%   83.25%   +0.55%     
==========================================
  Files         577      577              
  Lines       78288    81573    +3285     
==========================================
+ Hits        64749    67917    +3168     
- Misses      13539    13656     +117     

Files	Coverage Δ
...metry/Schemes/ProjectiveSchemes/Morphisms/Types.jl	91.30% <90.90%> (-0.13%)	:arrow_down:
...chemes/ProjectiveSchemes/Morphisms/Constructors.jl	76.34% <71.42%> (-0.41%)	:arrow_down:
.../Schemes/ProjectiveSchemes/Morphisms/Attributes.jl	80.20% <61.11%> (-4.41%)	:arrow_down:
src/Rings/MPolyQuo.jl	88.58% <21.42%> (-3.04%)	:arrow_down:

... and 66 files with indirect coverage changes

[fingolfin]

In triage, @jankoboehm was happy with this PR and @wdecker voiced no objections.

I suggest to address Simon's comment one way or another, and then get this merged RSN.