Change terminology borehole depth/length

[wouterpeere]

Currently, the property 'self.H' is used as the borehole length, but it is referred to as the borehole depth, which is another concept.
This issue has the goal of redefining the terminology, using self.H as the borehole length (as it is know) and defining another parameter 'self.depth' that is the actual depth of the deepest point in the borefield.

[wouterpeere]

In this issue, there is also improvement in the accuracy of temperature calculation with multiple ground layers and in sizing for all cases.

Currently, there was the assumptie for the temperature calculation that the average temperature was taken at depth H (with H the borehole length). For 99% of cases, this is correct, but if you drill under a building, the buried depth can become relevant for the average temperature. Therefore, all the temperature calculations of the ground will now take into account the buried depth as well.

For the calculation of the ground thermal conductivity and ground volumetric heat capacity the same reasoning is applied, where now the buried depth is taken into account.