Improve heading and track computation

[dharmab]

We currently use a naive implementation for determining headings and track directions by looking at the most recent position change of a single trackfile per group. This can give strange results in manuevering groups.

1. Incorporate all trackfiles in a group into heading and track direction determinating.
2. Look at the full trackfile length, not just the most recent two points. Fit a curve to the points and determine if a coherent track direction exists.

[dharmab]

(1) has been completed in #640 for SkyEye v2