Issue263 add borefield model

This is a pull request proposing a borefield model allowing arbitrary configuration of boreholes and taking they thermal interactions into account. The model is based on the step response of the borefield computed by two different models for the short-term and the long-term response, respectively.

The model uses an aggregation method to speed up the computations. An SHA-based algorithm is used to 
check if the step response and the aggregation matrix have already been calculated for the same parameter values. If not, this is done during the initialization of the model and saved in a temporary file.
add variable comments.