Integrand function inputs

[mrhardman]

Permit non-polynomial kernel functions in the construction of the finite element matrices.

This PR permits the calculation of matrices of the form

$$A_{ij} = \int^1_{-1} l_i(z) l_j(z) \rho(s z + c) s d z$$ $$B_{ij} = \frac{1}{s}\int^1_{-1} l_i(z) \frac{d l_j(z)}{d z} \rho(s z + c) s d z$$

(and the transpose $B_{ji}$) and

$$C_{ij} = \frac{1}{s^2}\int^1_{-1} \frac{d l_i(z)}{d z} \frac{d l_j(z)}{d z} \rho(s z + c) s d z.$$

The additional feature is the kernel function $\rho(x)$ with $x = s z + c$, and $z$ the reference coordinate in $[-1,1]$, and $s$ and $c$ the scale and shift factors, respectively. We specify the kernel function as an optional argument, e.g.,

coordinate = ElementCoordinates(x,scale,shift)
M = finite_element_matrix(lagrange_x,lagrange_x,coordinate,
            kernel_function=(v -> rho(v)),additional_quadrature_points=n)

where the integer parameter additional_quadrature_points permits the user to increase the number of Gauss-Legendre quadrature points to compensate for the increased complexity of the integrand due to rho(v).