Fix S curve acceleration issues

In some rare cases the S curve acceleration values are not computed correctly. Here is an example: let's say `accel = 0.186` then
```
// the following two lines are equivalent to the driver code
printf("A=%.*f\n", 2, accel);
printf("AA=%.*f\n", 2, accel/2);
```
prints
```
A=0.19
AA=0.09
```
which does not satisfy the 1/2A =< AA =< A condition and causes the following error:
```
*INVALID CONDITIONS FOR S_CURVE ACCELERATION-AXIS 1
```
See p. 12 of http://www.parkermotion.com/manuals/gemini/Gem6K/Gem6K_CR.pdf