customizbale flexing battery holders

Reworked SCAD file from https://www.thingiverse.com/thing:456900 by enif, adding support for thingiverse customizer and a range of options: more battery types, custom battery size, tweaking battery dimensions for that perfect fit, multi-cell packs, alternating polarities, battery labels, and battery contact type.

Various compiler warnings were fixed, and the code modernized and refactored.

The spring was changed to an internal double accordion type, since the thingiverse customizer wasn't able to handle the sinuous spline spring. (This is a bug in the version of the app thingiverse is using, not enfi's code.) The new spring is a bit weaker, but more accommodating of variations in battery lengths.

I've only directly tested AAA, AA, CR123A, 18650, and 18650P types. Other sizes were copied over, but if you find they are too big or small leave a comment. In the mean time you can always use the dimensional adjustments, under *Fine Tuning*. Note that the adjustment fields are ignore