Nft: tape-specific alphabets

[bruderjakob17]

Adds an option to have tape-specific alphabet pointers in an Nft.

The idea is that, if an optional vector of tape alphabet pointers is given, those are preferred when e.g. printing the Nft to dot format. A symbol is hence translated as determined by

• the tape-specific alphabet, if it exists (i.e. the vector of tape alphabet pointers is not nullopt, and there is a non-null pointer to some Alphabet at the current level), or otherwise
• the "generic" alphabet of the Nft, if it exists (i.e. it is a non-null pointer to some Alphabet)

meaning the tape-specific alphabet (if existent) is preferred over the generic Nft alphabet.

Also, this PR includes some modifications to keep the specific and generic alphabets during some common constructions.

[codecov]

Codecov Report

❌ Patch coverage is 40.47619% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.70%. Comparing base (b5f27b8) to head (3367c75).
⚠️ Report is 18 commits behind head on devel.

Files with missing lines	Patch %	Lines
src/nft/nft.cc	0.00%	19 Missing ⚠️
src/nft/operations.cc	40.00%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            devel     #612      +/-   ##
==========================================
- Coverage   72.90%   72.70%   -0.21%     
==========================================
  Files          45       45              
  Lines        6795     6829      +34     
  Branches     1538     1543       +5     
==========================================
+ Hits         4954     4965      +11     
- Misses       1227     1249      +22     
- Partials      614      615       +1     

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