Remove GPU package dependencies

[omlins]

The current state of Julia's extensions feature does not seem to provide a good way to make type aliases from the extensions available to the user (see #25 ). Given that type aliases are the only feature of CellArrays that actually require to import the GPU packages and therefore to create a dependency, we can provide the corresponding functionality using macros instead and then completely remove the GPU package dependencies.

[omlins]

CC: @albert-de-montserrat

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (1a4ed60) 63.56% compared to head (b701ea6) 74.38%.

Files	Patch %	Lines
src/CellArray.jl	80.00%	2 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##             main      #26       +/-   ##
===========================================
+ Coverage   63.56%   74.38%   +10.81%     
===========================================
  Files           3        3               
  Lines         129      121        -8     
===========================================
+ Hits           82       90        +8     
+ Misses         47       31       -16     

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