Customizable V-Block (Rigid)

This is a tweak of gmelenka's v-block design, that allows a bit more clamping force.

Changes:

* One of the 'V's have been removed, adding rigidity
* The clamp grooves have been moved up 10mm, which allows the bottom half of the v-block to be secured more deeply into the vice.
* The clamp groove height is now configurable, with the variable 'sideHeight'.

Instructions:

* If you've got a slicer with decent support generation, like Simplify3D, try printing the v-block with the down-sid