Parametric roller bearing

# Fully parametric roller bearing

Non-print-in-place, i.e. the parts are printed separately and assembled. While print-in-place bearings makes for a nice 3d printing demo, they must necessarily be rather loose to not fuse together. If it instead is assembled from parts it can be made really tight, almost approaching a real bearing.

## Printing

No support. Print the rollers with a brim if you have problems getting them to stick to the bed. Small bearings could have this problem, larger shouldn't. Set 'seam position' to random to make the parts as round as possible. If you have problems with an 'elephant foot', i.e. the first layer sticking out due to being squashed against the plate, then you might need to trim it away from the roller ends and the cage holes so they roll freely inside the cage. Any obvious blobs should be trimmed away from the races and the rollers before assembling.

## Assembling
1. Put the rollers side by side against the outer ring. Then fit the inner ri