Update the python script for the BHE Array 2D benchmark

In this MR. The python script of the analytical solution to calculate the temperature distribution within an multiple BHE array is updated, since the previous python script is out-of-date and can not be further executed. The python script is for the interested readers who would like to try the superposition principle based analytical solution for a BHE array system proposed by Bayer et al. (2014). Currently there is no individual test for this python script.

Besides, in the related benchmark documentation, the typo error for the input parameter is modified. And the display error for the equation 4 in the benchmark is also fixed.