let exponent in station weighting function be a user option or else station density or terrain dependent #33
Description
The station weighting function is hardwired to an inverse cubic. This may exclude (by downweighting) too many stations in some case, thus having the exponent be data-responsive or user controlled may be beneficial. The flexible gaussian-esque form may be sufficient without changing further. Along with the user specified max distance, this is effectively the same weighting fcn as in Werner et al (2004), with alpha & lambda params.
The current 'USE_STATION_WEIGHTING' could be generalized/eliminated with this option, since specifying an exponent of 0 means there is no station weighting; an exponent of 1 means it is linear with distance.