Correct calculation of volume within storage node when using porosity

Currently when a porosity value is applied to a storage node, the porosity is only applied for storage array entries which are below ground level. Storage array entries above ground level are assumed to have a porosity of 100%. Between the highest storage array level value below ground and the lowest storage array level value above ground the porosity is assumed to increase linearly between the value you applied and 100%. i.e. the storage is assumed to be conical. This should be corrected. The porosity value applied should be applied to the whole storage array both above and below ground. An improved version beyond the above correction would be the ability to vary porosity throughout the storage. For example, a SuDS bioretention unit, or pervious pavement would have variable porosity with depth.