adding parametric shape for sphere

[shimwell]

User request for spheres, these could also be useful for plotting the source particles.

my_model = NeutronicsModel().export_stp() could export source points

This is how cadquery makes spheres

import cadquery as cq
import paramak

sphere_small = cq.Workplane("XY").sphere(10)
sphere_big = cq.Workplane("XY").sphere(20)

hollow_sphere = sphere_big.cut(sphere_small)

my_shape = paramak.Shape()
my_shape.solid = hollow_sphere

my_shape.export_stp('thanks_for_the_idea_steve.stp')
my_shape.show()

So we could make a new paramaic shape like this ...

def Sphere(Shape):

    def __init__(
        position,
        radius,
    ):
...