Add note on non negligible cost of finite differences to tutorial on shape derivatives

[oskooi]

Adds the following note to Tutorial/Derivatives with Respect to Shape Parameters:

A finite difference involves two function evaluations of $\rho(h)$, but the cost for this is negligible since it involves no Meep simulations. However, in cases when there are a large number of shape parameters and a dense design grid, the cost of the finite differences can be non negligible: $M$ shape parameters require $M$ finite differences each involving $N$ grid points.

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All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2883      +/-   ##
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- Coverage   73.81%   73.78%   -0.04%     
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  Files          18       18              
  Lines        5423     5428       +5     
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+ Hits         4003     4005       +2     
- Misses       1420     1423       +3