Footing class refactors to support combinated footing design

[mapra99]

Some findings:

1. We need to compute the effective height for a given straight_longitudinal_reinforcement instance
2. We need to compute the reinforcement ratio for a given straight_longitudinal_reinforcement instance and afferent_width instead of assuming that the width is going to be the whole perpendicular direction length
3. We need to support passing multiple straight_longitudinal_reinforcement instances to each reinforcement argument in the footing constructor. Investigate what else could break because of this