Changed seeding threshold to be equal to tracking threshold

Previously, the default FA/FOD amplitude threshold when finding a seed point was twice the threshold used during streamline propagation.
This has now been changed to be equivalent to the tracking threshold.
This was deemed to be more appropriate given recent changes to how the seeding is performed.
Variable maximum number of tracking attempts from seed point

As discussed in Issue #41.
Once an appropriate seed point is determined, a probabilistic tracking algorithm may take some number of attempts to find a suitable direction to commence tracking.
The maximum number of attempts now depends on the mechanism used to generate the seed points.
For instance, if the seed points are drawn randomly from a mask image, only 1 direction is tested per seed point; whereas if seeds are defined on a lattice grid, every attempt is made to find a valid tracking direction from every seed point.
This behaviour can be overridden using the -max_seed_attempts option.
Added -stop option to tckgen

Closes #40.
Just as in MRtrix 0.2, the new -stop option instructs the tractography algorithm to cease streamline propagation as soon as a given streamline has entered all inclusion regions.
A warning will be produced if the user tries to apply this option but does not provide any inclusion regions.
This option cannot be used in conjunction with back-tracking in ACT.