Added tick to inch conversions to encoder.c

Also added tick to centimeter conversions. Consider replacing some or
all of these with macro functions instead.

To do: update header file to include these functions
Added movement-specific functions to ruler system

We now can get the drive ticks, strafe ticks, rotate ticks, and swing
ticks for rulers. Any of the functions can be used while doing any
movement, but an unrelated calculation will yield a very  wrong result.
In most cases, the result will likely be zero.
Removed fields from the timer structure

The only field in Timer_t is now milliseconds, and any other measurement
of time can be accessed by using a function whenever it is needed. This
way will save lots of unnecessary memory from the fields, and will make
the program faster by not calculating things then not necessary.