Structural elements: implementation of truss element

[ahojukka5]

This is already started in #176. Aim is to implement a simple truss element for linear elasticity. Take a look also to the JuliaFEM.org/examples. Let's put the focus on static analysis first. This can later on be extended to dynamics also.

• [ ] Finalize implementation. We should have working truss model for linear elasticity, for 2d and 3d. It should take at least Young's modulus and cross-section area in order to work. Make it work for both linear elements (Seg2) and quadratic elements (Seg3).
• [ ] Create a test. Testing of model should be done with some known result. Testing must be done for 2d and 3d models separately. We can calculate some simple truss structure using e.g. ABAQUS or Code Aster and compare results to that.
• [ ] Create Jupyter notebook. It can contain some simple introduction part with review to the theory. Then we show how to solve simple truss problem using JuliaFEM. This goes to http://www.juliafem.org/examples
• [ ] Optionally, instead of introducting theory in Notebook, a separate .md file can be created for Truss elements and included to docs/src/theory/truss.md

[ahojukka5]

This is on progress on FEMTruss.jl