Add functionality to show physical values distribution as contour plot: nodes

[gabrielpreda]

Add functionality to show physical values distribution as contour plot: nodes

Currently the package import an FEM file and display (using plot2D) the geometry of the FEM
Typically, FEM is used to calculate a physical model and output will be one or more physical values, distributed in the FEM mesh; for example, in electromagnetics will be the field density, electric potential, in mechanics will be the force, the stress distribution etc.

We would like to add functionality to show such physical model values distribution in the FEM mesh.
This could be either nodes values of element values.
In this case, we would like to show nodes value.
Nodes value will show a value of color interpolated between nodes values in each element. The color interpolation scheme should be linear.
A legend showing the physical magnitude plotted is as well requested.