Improve residual gas scattering calculations

This allows you to specify the partial pressure of each species as a function of position along the beamtubes instead of assuming that they are constant. An example is given in test/noise/residualgas/AplusSuperGas.

The original gas scattering calculation also approximated the integral by expanding the exponential to first order. This approximation is suspect and has been replaced with the same numerical integral that is needed for arbitrary pressure profiles. The following two plots show the difference between the new exact calculations and the old approximate ones for A+