downgrading precision on fill_Point

[vfonov]

https://github.com/BIC-MNI/libminc/blob/develop/volume_io/Include/volume_io/geom_structs.h#L69

Accoriding to Claude Lepage, this macro is used in many places with double precision data , but it explicitly converts values to float , causing loss of precision.

[gdevenyi]

Dumbest solution, convert to double:
https://github.com/BIC-MNI/libminc/blob/develop/volume_io/Include/volume_io/geom_structs.h#L51

If VIO_Point_coord_type is being used consistently, it should upgrade everything.