Added SBR Rails

[jeroenrnl]

Issue #240 This change adds

• SBR rails. For now only SBR16
• The bearing block that goes with it (SBR16UU). I made a split in bearing_blocks to support a new kind of bearing block. I tried to reuse some code, but due to the fact that the letters used to indicate measurements are vastly different between SBR and SCS - well and the fact that the blocks are quite different as well - this was only possible in a very limited way.
• An 'open' linear bearing (LM16UUOP) that is used by the bearing block, I made some changes in the linear_bearing[s].scad to support this.
• A 28mm circlip that has a wider opening than then normal circlip. (circlip_28iw)

It should be fairly easy to add the other types of rails, bearing blocks and linear bearings, but I first wanted to hear whether my current approach is the right one. I used this PDF, as well as a set of SBR16 rails I had on hand and several other online drawings I could find.

The 28mm 'wide' open circlip is something I could not find any documentation on, so the measurements are mostly guessed, based on the one that is in the SBR16UU I have. The name 'circlip_28iw' is something I made up.

[jeroenrnl]

I also rebased the branch on master so it has the changes you made in the past few days incorporated.

[nophead]

I took the liberty of deriving the open angle from the open width. I can't see how they can not be exactly related. On the data sheets the numbers vary a bit but that seems to just be rounding as only one can be an integer.