Add sound-soft CFIE

[jackhamel16]

Theory is complete and can now try implementing this

[jackhamel16]

Needs:

• [ ] rename sound-soft IE routines to distinguish them from new cfie routines

RHS:

• [x] same routine as used for sound-soft IE
• [ ] normal derivative of incident field in same routine as used in sound-soft IE
  • [ ] normal derivative of incident fields Matrix:
• [x] term 1: Test integral over src basis times test basis functions
• [x] term 2: Original sound-soft IE routine
• [x] term 3: Principal value integral. Test and src integral over both bases times normal derivative of greens function
  • [x] normal derivative of greens function

organization: Defining a solve routine for each IE I add seems like the best approach. Within the solve routine, call rhsFill and matrixFill for each integral described above and add results to get the final matrix and rhs.

multiple dispatch on fill routines so instead of allocating matrix/rhs inside the functions, it is passed as an argument and the elements are added to matr/rhs entries in the routine. Would allow multiple calls with different integrands (e.g. the two terms in the cfie RHS)

[jackhamel16]

Normal derivatives of spherical wave excitation will be nontrivial. Here are some sources to get going with:

https://mathoverflow.net/questions/301827/partial-derivatives-of-spherical-harmonics https://en.wikipedia.org/wiki/Spherical_harmonics