add spar cap width/angle based blade structure component

[fzahle]

the blade structure parameterisation is now generalized and there's no method to specify the chordwise position of the spar cap, its angle relative to the rotor plane and width. a component that either simply computes these properties which could then be used for constraints is one option, another option is to have these quantities available as design variables.

[fzahle]

Related to this, create a setup method for defining the blade structure from scratch which at present is not so easy. the method should add necessary regions and webs:

setup_bladestructure(nr=11, nw=3)

and placement of region boundaries should be based on the chordwise fraction, since the surface fraction is not intuitive.

for placement of the spar cap, the user should be able to specify e.g. maximum thickness location or chordwise fraction as well as angle.