[FTheoryTools] Fixed some bugs that were missed before

[emikelsons]

Some small bug fixes that were missed in some recents pull requests relating to model indexes and zero section classes of FTheory models:

• model_index doesn't result in errors anymore(there might have been a defunct method that was used before)
• Model constructors now no longer attempt to set a zero_section_class for models constructed over an arbitrary base
• For Tate and Weierstrass models the zero section class is now always set to be 'z'

[emikelsons]

@apturner @HereAround

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 84.65%. Comparing base (759becf) to head (96b0331).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
.../FTheoryTools/src/LiteratureModels/create_index.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4182      +/-   ##
==========================================
+ Coverage   84.58%   84.65%   +0.06%     
==========================================
  Files         631      631              
  Lines       84829    85271     +442     
==========================================
+ Hits        71756    72184     +428     
- Misses      13073    13087      +14     

Files with missing lines	Coverage Δ
.../FTheoryTools/src/AbstractFTheoryModels/methods.jl	78.27% <ø> (ø)
.../FTheoryTools/src/LiteratureModels/constructors.jl	93.85% <100.00%> (+0.04%)	⬆️
...rimental/FTheoryTools/src/TateModels/attributes.jl	86.58% <ø> (ø)
...l/FTheoryTools/src/WeierstrassModels/attributes.jl	86.53% <ø> (ø)
.../FTheoryTools/src/LiteratureModels/create_index.jl	0.00% <0.00%> (ø)

... and 5 files with indirect coverage changes