Given three points in 3D space, returns the triangle area.
Assumes:
points are given as numpy arrays (crashes if not met)
if points lists are given, this will still work
Parameters :
p1, p2, p3 : numpy.array
If a list of points is given, they must be vertically stacked.
Returns :
area : scalar / numpy.array
The area of the points defined by the triangles. If lists of points
were used as inputs, the output is a 1D numpy.array with as many
elements as given points