tracker/pt: declare floating-point type size in one place

We want double precision for POSIT. It's best for the type to be set in
ope place without the need to go over everything while switching it back
and forth during tests.

Machine epsilon for float is very small as per
<https://en.wikipedia.org/wiki/Machine_epsilon>. Also see the absurdly
high epsilon of 1e-4 of POSIT that we've had. With floats, making the
epsilon lower resulted in change deltas flushing to zero. This typically
led to the translation Z value being very unstable in PT.

After the epsilon and data type size changes the Z value is stable.

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