define FusionTensorAxes

[ogauthe]

This PR defines FusionTensorAxes to handle separately axes logic from FusionTensor properties. It also reorder and clean files.

[codecov]

Codecov Report

Attention: Patch coverage is 99.02913% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.00%. Comparing base (7ccd853) to head (97a4ebc).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/fusiontensor/fusiontensor.jl	98.03%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #58      +/-   ##
==========================================
+ Coverage   90.41%   91.00%   +0.59%     
==========================================
  Files          10       11       +1     
  Lines         532      556      +24     
==========================================
+ Hits          481      506      +25     
+ Misses         51       50       -1     

Flag	Coverage Δ
docs	0.00% <0.00%> (ø)

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