VPMicroCluster: Define setters for interstrip fraction from uint8_t instead of float

See discussion in Moore#793 (closed)

Setters from float where ill defined, due to the implicit conversion from a 32 bit float to a 8 bit char type. These in fact appear not used so instead define setters from the actual fract type as saved in the clusters, an 8 bit unsigned type.

Small additional clean up at the same time.