Incorrect linear acceleration function code

[mxgrey]

I noticed that this acceleration function has the same source code as this velocity function. This is most likely a copy/paste error.

This version of the function does not get tested in testDynamics (instead, the no-argument version is tested), which is probably how this has gone unnoticed. For now, it's probably sufficient to just change each mV in the function to mA. The upcoming code where I created the Frame class already has tests for all of its different position, velocity, and acceleration functions, so we shouldn't have to worry about writing new tests to catch this stuff.

[mxgrey]

Fixed by #319