Add ScalingPowerFilter to the project.

ScalingPowerFilter allows applying an arbitrary x^c scale to all axes
on a gamepad.
Remove unnecessary gamepad filters.

The QuarcticScalingFilter and ScalingFilter classes are no longer needed
with the new ScalingPowerFilter.
Add ScalingLinearFilter to the project.

ScalingLinearFilter allows applying a linear filter c*x to all axes
on a gamepad.
Modify operator rotation controls to use ScalingLinearFilter.

The robot operator now has linear scaling with all rotation control
scaled down by 0.5.
Modify Drivetrain subsystem encoder code to reflect the two encoders.

Renamed getAverageVelocity method to getAverageForwardVelocity to
reflect the fact that this method only computes forward velocity.
Update robot components to reflect the new Drivetrain constructor.

Drivetrain only takes channels for two encoders instead of the
originally planned four encoders.
Update DriveForXDistanceCommand to use the renamed Drivetrain methods.

DriveForXDistanceCommand uses Drivetrain.getAverageForwardVelocity
and drives forward at a given constant power.
Update the Conveyor subsystem to include different speed powers.

The new speed powers allow different conveyor speeds for the driver
and the operator reflecting their different uses for the conveyor.
Update robot to use the new Conveyor speed constants.

The different robot operations now use the appropriate conveyor speed
constants.