[ENH] Add rectangular solid sections to the section manager

**Is your feature request related to a problem? Please describe.**
Sometimes it is necessary for structural engineers to calculate the section capacity of solid rectangular plate sections such as gusset plates. Currently eurocodedesign only contains standard sections with predefined geometry.

**Describe the solution you'd like**
The inclusion of a class for rectangular solid sections where the height and width of the section are provided. e.g. `RectangularSolidSection(h, b)`

Upon instanstiation the RectangularSolidSection class will, based on the provided height and width, calculate all of the appropriate cross-section properties.

The section should inherit from `SteelSection`