Remove access of .data field on abstract types

[akirakyle]

Fixes #12 for .data field access. Implementing #40 will fix instances of basis{_l,_r} access.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.

Project coverage is 35.15%. Comparing base (f9171c6) to head (4f3571a).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/julia_base.jl	0.00%	5 Missing ⚠️
src/julia_linalg.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #57      +/-   ##
==========================================
+ Coverage   34.66%   35.15%   +0.48%     
==========================================
  Files          15       15              
  Lines         424      421       -3     
==========================================
+ Hits          147      148       +1     
+ Misses        277      273       -4     

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[Krastanov]

I added a small comment explaining the custom error functions because they confused me a bit. Thanks for submitting this. I will merge this and the related one shortly.