Sectionproperties

- Added calculation of monosymmetric constants
- Added tapered flange I-section and channel sections
- Added solid elliptical and hollow elliptcal sections (BenjaminFraser)
- Added polygonal section (Agent6-6-6)
- Handle zero radius for all section classes; handle r_out < t for relevant sections
- Update Cee and Zed sections to account for short lips

- Added a monosymmetric I-section class
- Extend the plastic centroid search range to the entire section
- Remove the pc_region variable from the plastic centroid calculation as it is no longer relevant
- Better verbose output for the plastic centroid calculation

- Retrieve cross-section stresses using get_stress()

- Fix returns for adding to geometry

- Initial release of the package rebuild.