give user control of filtering before map-matching

[kodonnell]

As discussed starting here, we may want to allow users to be prevent all filtering (and hence include all input points in the map match result).

A second possible use case - one may want the 'https://github.com/graphhopper/map-matching/blob/master/matching-core/src/main/java/com/graphhopper/matching/MapMatching.java#L259-L261' to be greater than 2 * measurementErrorSigma - to e.g. 'smooth' the result, or maybe for testing quality i.e. #89.

[INRIX-Trang-Nguyen]

I've added a separate configuration parameter called gpxFilterThreshold which is applied to filterGPXEntries (rather than 2*measurementErrorSigma), We have a need to tune the level of filtering of GPX points prior to map matching. It is a simple change but I can submit a pull request if someone can give me access.
Thanks,
Trang