favor type usize for variables that are primarily an array index

[stonylohr]

Type usize is required for the index into an array or slice. It is generally preferable to go ahead and use this type from the outset for variables for which this is the primary purpose, to reduce the need for casting later. Unless there is a specific reason to do otherwise, of course.

Examples of places that might benefit from this change...
geodesic: GEODESIC_ORDER
geomath::_A1m1f: geodesic_order
geomath::_C1f: geodesic_order, m, o
geomath::_A2m1f: m
geomath::_C2f: o, m

[michaelkirk]

SGTM

[valarauca]

Fixed in #58

[michaelkirk]

In #57