Integrate wavefields for height gain functions

[fgasdia]

Currently the Modified Hankel Functions of Order One-Third are used to compute the height gain functions (and excitation factors) for the mode sum. Consider integrating the wavefields directly. This may be easier to extend fields into the ionosphere and above.

As part of this, study the accuracy of the Modified Hankel Functions relative to the full wave solution.

The increase in runtime will be negligible relative to mode finding.