Issue196 borehole

Hi Micheal,

i review the code:
1) I accepted your changes in convectionResistance. However, I do not fully understand why you consider that the conduction resistance is approximately equal the the convection one for low mass flow rate (the correlation for the heat transfer function is then far off its boundaries) (in regZeroPower).
2) in singleUTubeResistances, it was not correct to take the convection resistance into account, as this function computes the conduction resistances within the borehole. So now, they do not depend on the mass flow rate anymore, and the slicefunction can be avoided.

You also asked to add the picture for the documentation. In which folder should I put them?
I put them here in attached file:
![bauer_singleutube](https://f.cloud.github.com/assets/2987659/2151014/ebe3f952-9406-11e3-90a7-e44678ffc668.png)
![bauer_singleutube_small](https://f.cloud.github.com/assets/2987659/2151015/ebe7d004-9406-11e3-9991-a16dcb3eb1e5.png)
![ra_multipole](https://f.cloud.github.com/assets/2987659/2151017/ec015894-9406-11e3-8236-7e6b846512d7.PNG)
![rb_multipole](https://f.cloud.github.com/assets/2987659/2151018/ec068468-9406-11e3-8f9f-310017d5edd3.PNG)
![bauer_resistancevalues](https://f.cloud.github.com/assets/2987659/2151019/ec1e6a88-9406-11e3-96a9-b3fdf8bc334e.PNG)

let me knwo if other modification are necessary.