Added new Shape - Helical Ramp

New shape Helical Ramp draws a ramp with parameters to specify start and end radius, start and end width of ramp, start angle (relative to x-axis), vertical Pitch, and number of rotations along z-axis. Both Pitch and Number of Rotations can be negative: negative Pitch draws the helix downwards; negative Number of Rotations draws a left-handed helix.

DOCUMENTATION.md and CHANGELOG.md updated for new Shape.