Groundwater advection

[MassimoCimmino]

This issue is to implement the moving infinite line source solution (MFLS) for the evaluation of g-functions with groundwater flow. The solution is presented in [1]. An inclined moving finite line source should be possible following the implementation in #50. One thing to keep in mind is that, under groundwater flow, the reciprocal thermal response factor h_12 != H_2/H_1 * h_21. Similarity rules can be inferred from the MFLS solution. The 'equivalent' solver may be extended to consider groundwater flow. It is unknown if an approximation of the MFLS is possible.

The heat transport velocity can be added as an optional parameter to the finite_line_source function and the gFunction class, with v_T = 0..

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[1] Molina-Giraldo, N., Blum, P., Zhu, K., Bayer, P., & Fang, Z. (2011). A moving finite line source model to simulate borehole heat exchangers with groundwater advection. International Journal of Thermal Sciences, 50(12), 2506-2513.

[wouterpeere]

There are a couple of improvements I think would be possible to make to the model, which are not explicitly in the paper:

1. Add a flow direction, so the orientation of the borefield w.r.t. the flow can be considered.
2. Add the flow only to a certain part of the borefield. Typically, you only have a flow at certain depths in the ground and not in certain aquitard layers. This could be an interesting addition, where you can set the range where there is flow.

I'm not 100% sure about the complexity of these additions, but since the borehole is segmented, I think it would be feasible.