assert hinted attrs are bound and have right dtype after initialisation

[jsouter]

Closes #125
Closes #124

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.25%. Comparing base (ff0be1d) to head (91a7c41).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/fastcs/util.py	94.44%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #187      +/-   ##
==========================================
+ Coverage   92.19%   92.25%   +0.05%     
==========================================
  Files          40       40              
  Lines        2012     2040      +28     
==========================================
+ Hits         1855     1882      +27     
- Misses        157      158       +1     

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[GDYendell]

Just a couple of comments.

I don't love that this check is done by Backend, but I think this can be improved later. When we create an add_attribute method for creating dynamic attributes that would be a good hook to do this check.