Parametric battery holders with selectable pre-sets (Flexing with integrated spring)

Just a rework of the great flexbatter scad code ( @enif https://www.thingiverse.com/thing:456900) to enable selection of a larger number of pre-set variants **without** having to edit the code.

(Great for recycling the 13400 battery in disposable vape pens)

If **any** pre-set other than **custom** is selected, **all** parameters are ignored.

To add/delete pre-sets, simply edit the variable 'create_this_preset' and then add/delete a corresponding entry in the list 'pre-sets'

the entries in the list are in the same order as they are required in the module 'flexbatter'
it currently includes pre-sets for the following


AAAx1,1xAAA,AAAx2,1xAAAx2,AAAx3,1xAAAx3,AAAx4,1xAAAx4,2xAAA,
2xAAAx2,3xAAA,1xAA,2xAA,AAx1,AAx2,1xAAx2,AAx3,1xAAx3,AAx4,
1xAAx4,2xAAx2,3xAA,CR123Ax1,3xC,Cx1,1xC,1xCx2,1xCx3,1xCx4,2xC,
2xCx2,Cx2,D,18650Px1,18650Px2,18650Px3,18650Px4,18650x1,1x18650,
18650x2,1x18650x2,18650x3,1x18650x3,18650x4,1x18650x4,2x18650,
2x18650x2,26650x1,26650x2,26650x3,26650x4,13400x1,2x13400,134