add spring supports

**Is your feature request related to a problem? Please describe.**
No. Just more wish list stuff :)

**Describe the solution you'd like**
Spring supports.

**Additional context**
One of the uses I have in mind for xlFrames is modeling treated wooden posts embedded in the ground, a.k.a. post frame foundations. Current best practice is to model the soil's lateral capacity as a series of springs with k-values determined by geotechnical properties of the various soil horizons, something like this...

When analysis software lacks spring supports these soil springs are modeled as additional nodes and members (with AE/L = k_soil_spring) - not the end of the world - It's simple enough to set that up, just a bit of a hassle to exclude these extra nodes and members from the desired results. Built-in spring supports would be a welcome upgrade.