kernels as a class

[askhamwhat]

create a kernel class to allow for more sophisticated kernel handling. e.g. an instance of the kernel class could have preferred methods for:

• kernel splittings for use in Helsing-Ojala style quadrature
• fast kernel evals, e.g. by calling fmm2d wrappers
• stable kernel evaluation for small inputs
• evaluating gradient, hessian etc. constructors should be built for the common PDEs: Laplace, Helmholtz, Stokes, etc, with default kernels by boundary condition, etc.

[askhamwhat]

this was implemented by Dan Fortunato and merged into master