docs: consistency of variable names

[mancellin]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.93%. Comparing base (597cc23) to head (dad5a8d).

Additional details and impacted files

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##             main     #103       +/-   ##
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- Coverage   79.97%   65.93%   -14.04%     
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  Files          24       24               
  Lines        3021     3018        -3     
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- Hits         2416     1990      -426     
- Misses        605     1028      +423     

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[maltezfaria]

Thanks @mancellin . I realised we are (also) not being consistent with the four letters used for the BIEs.

I guess we should pick a convention and stick with it. @tanderson92 and @caperezar suggested S,D,K,N as in Nedelec's book. Would you mind changing those in the integral_operators.md file? ]

I can do it as well and push to this branch if you prefer.

[mancellin]

Would you mind changing those in the integral_operators.md file?

Unless I've missed something, integral_operator.md already uses S,D,K,N consistently with this PR.

[maltezfaria]

D' is used in some of the formulas to mean K I believe...