Added some shapes

Added Octagon, Hexagon, and prisms of each. Also added a rounding
function for use in octagons and hexagons
removed the n-gon prism functions

Will add them back in in a similar manner to the cylinder.
Added new shapes to ChoiceFunct()

Added a new page to the selector menu and added if statements for each
of the 4 new shapes. I copied these ifs from the circle (and cylinder)
if and changed variable names to suit the n-gons.