Reduce amount of calculation for exact solution

[AbdelhadiKara]

Compute exact solution vector once, store it on host and pass it into functions instead of passing ExactSolution instance.

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[codecov]

Codecov Report

❌ Patch coverage is 93.54839% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.74%. Comparing base (a87782a) to head (26da51e).

Files with missing lines	Patch %	Lines
include/GMGPolar/solver.h	93.54%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #277      +/-   ##
==========================================
+ Coverage   95.73%   95.74%   +0.01%     
==========================================
  Files          79       79              
  Lines        8492     8512      +20     
==========================================
+ Hits         8130     8150      +20     
  Misses        362      362              

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[julianlitz]

I would not recommend to use The .multiIndex.
I see the benefit that you then only need a single loop to achieve the same access pattern, but you do need more expensive integer operations to compute your indices.

I think that is the main reason GMGPolar can be so fast because we dont need to spend ages converting global to multiindices and searching for neighboring indices.

[julianlitz]

You could check your solution against #270 if you want

[AbdelhadiKara]

@julianlitz @EmilyBourne I tried to take into account all of your comments. Feel free to comment if you still see something needs modification.