Ford: higher curvature rate limits at low speed

[sshane]

panda PR: commaai/panda#2065

The low speed limits imposed by the EPS are still insanely low (< 0.5 m/s^3 at 7 m/s), but this still is an improvement from our current limits. In the segment below, we increase our rate limiting ~2x to match the EPS's limits:

interp(7, [5, 25], [0.0002, 0.0001]) * 20 * 7 ** 2
Out[48]: 0.18620000000000003
interp(7, [5, 25], [0.00045, 0.000125]) * 20 * 7 ** 2
Out[49]: 0.40915

# EPS at this speed: 0.009 * 7 ** 2 = 0.441

An example of stock EPS limiting us: