Calculate photophoretic force

[myurkin]

This force is caused by non-uniform distribution of temperature over the particle surface, however, it can be expressed (in different regimes) through volume integrals of absorption density (internal field intensity) multiplied by a special weighting function. This integrals are easy to implement in ADDA, but weighting function is known analytically only for a number of simple shapes. For other shapes, an adjoint problem need to be solved. It is relatively simple, but another code is required for that. So the feasible first solution may be based on user providing such function through an input file. Fortunately, it depends only on the particle itself (similar to the Green's function) - after it is known any number of incident beams can be considered.