Refactor input arrays/vars to C structs

[unrealsolver]

Structs are easier to work with, and this will allow us to map them to JSON and back later.

Subtasks:

• Remove old separated variables and replace them with new structs #7
• Nodes/Edges lists should have uids #12

Frame data:

• Nodes list
• DoF
• Edges list

Load case data:

• Point loads
• Uniform loads
• Trapezoidal loads
• Internal concentrated loads
• Temperature loads
• Prescribed displacements
• Dynamic load cases

Simulation options:

• Simulation options

Root struct:

• Root struct