Add support to load multiple Manifold Solids

[hideak]

The following code was obtained from the Interface.ImportMenu module and is responsible for loading an .IGES file into the application. This part of the code uses a previously created Reader object to call an internal method named TransferList() to obtain all the loaded entities of the model, including all Manifold Solids, Faces, Surfaces, Edges, Vertices and other entities:

# Checking if the file were read successfully and creating a OpenCASCADE shape object:
shape = None
if status == IFSelect_RetDone:
    Reader.TransferList(Reader.GiveList('xst-model-all'))
    for i in range(1, Reader.NbShapes()+1):
        parent.shapeList.append(Reader.Shape(i))
    shape = Reader.Shape(1)

However, the shape variable only stores the first loaded shape: Reader.Shape(1) and thus, the following code:

# Displaying the loaded entity as an OpenCASCADE shape:
parent.canvas._display.DisplayShape(shape, update=True)

Just display the first entity, even if the loaded .IGES file contains multiple Manifold Solids.

The aim of this Issue is to implement a logic to load as many Manifold Solids as needed, so all of them can be used by the discretization methods.