More light-weight truss ground structure IO

[yijiangh]

Currently, we use a JSON file to parse ground structures for truss problems. However, this is entirely for the convenience of debugging and unit tests. The json file can get unnecessarily large when the ground structure gets denser.

Here are some thoughts:

• We might consider using LightGraph to represent graphs and use their IO methods.
• We should have some standard problem struct for truss problems, similar to the PointLoadCantilever and L-Beam for solid problems. For example, we should at least have a ground structure method for distributing points (parameter 1) in a cube-like domain, and connect each points' k neighboring points (parameter 2).