Add two new reworked Fusion templates for creating centerpieces

[ColinCampbell]

I was using the previously available centerpiece template to print some attachments but I noticed the fit between the printed pins and the sidepieces I had printed was really poor. In digging into the Fusion source, I found that a lot of the measurements were slightly off:

• The pins defined defined as 9.5mm in length, when they are supposed to be 10mm - 0.1mm with the 0.1mm being removed for easier fit. A similar issue exists for height at 3.5mm instead of 4mm - 0.1mm
• The offsets for the pins left the printed centerpiece offset from its sidepieces which looked weird and unintentional
• The thickness was likewise leaving gaps

In taking a look at the more up-to-date FreeCAD source, it had:

• Different measurements which from my checking against the SketchUp sources would exactly match up with the designed sidepieces
• A slightly different approach of generating a centerpiece that had the pin holes designed into it

This pull request adds two different Fusion templates, one which will generate a centerpiece that has the pin holes in it, allowing you to print the pins separately, along with one that generates the pins into the centerpiece itself. I mostly did the former to copy the approach of the FreeCAD source and then decided that having an updated version of the old template would also be helpful. Let me know if you'd prefer to just have one!

The existing pin-included template had a fillet on the centerpiece but the FreeCAD version didn't. The sidepieces I have printed don't have a rounded edge so a flat edge looks better. I was trying to figure out how to get Fusion to make the fillet configurable but couldn't get it. I figured people could do what I did and just delete the fillet if they don't want it but wanted to see what you thought first.

I've tried to clarify some of the parameters to make them easier to understand. I've attached screenshots of the parameters as well as what is generated for review. Happy to adjust anything.

[ColinCampbell]

I should mention that I printed a test piece from each of these and verified that the fit the existing sidepieces I have (2x0 Flat Left/Right) and they mount perfectly to the wall board when I was having trouble with the previous template