Improve meshing and snapping points in lumped elements

[weiliangjin2021]

Still many tests to fix and add, and CHANGELOG to add, but @dmarek-flex could you take a look first to see if the change makes sense?

Two main changes:

• Meshing part: snapping points along normal axis, and one override structure for the entire lumped element, with default grid number to 1.
• Mesh is generated automatically by moving lumped element meshing to grid_spec. The generated override structures and snapping points are considered internal.