Issue263 add borefield model

Here is the borefield model.

I reverted all the unwilling changes that happened on the Buildings models. I hope I did it correctly.

Any comments or questions are welcome.
I see, based on some search, it looks like erf.c is not installed on the default search path for some Windows compilers.
I recommend you just convert erf.c from c to Modelica as it is only a few lines long.

Michael