Added interpolation envelope function in "Simulations"

A new function "int_envelope" is added in Simulations to interpolate linearly the closed contours from the convex hulls in a defined amount of points (default set Nx=300).

New additions:
- "int_envelope" function;
- flag "int_env"(set as False) in the "compute_envelope" function; if the flag is set as True, the computed evelope is interpolated in a given set of points. 

See merge request !8