changed Buildings.Fluid.Movers.BaseClasses.Characteristics.power:

Hi,

I made the following changes:
Similarity law now uses a working point based on a rotational speed V_flow/r_N instead of V_flow. Only when referring to this working point then the power law P2= r_N^3*P1 is valid.

I have some images that can serve as a limited validation. The graphs you see are from the data sheets from real pumps. 
Before:
![before1](https://f.cloud.github.com/assets/5970879/2256545/d370374e-9e08-11e3-84e7-e5253738b153.png)
![before2](https://f.cloud.github.com/assets/5970879/2256549/d372f01a-9e08-11e3-951f-e85149077f49.png)
![before3](https://f.cloud.github.com/assets/5970879/2256547/d37108d6-9e08-11e3-81e5-e08c2c86bcaf.png)
After:
![after1](https://f.cloud.github.com/assets/5970879/2256543/d3558b74-9e08-11e3-91f9-42a55afb4a43.png)
![after2](https://f.cloud.github.com/assets/5970879/2256548/d372b76c-9e08-11e3-88f9-94431526614f.png)
![after3](https://f.cloud.github.com/assets/5970879/2256546/d370759c-9e08-11e3-99e9-a9cbb424a715.png)

Also, for the following figures I changed the power law from:
P:=r_N^3*Buildings.Utili ....
to 
P:=r_N^2.75*Buildings.Utili ....

This seems to give even better results:
![power1](https://f.cloud.github.com/assets/5970879/2256544/d3679c92-9e08-11e3-8e99-87c1d9b462b5.png)
![power2](https://f.cloud.github.com/assets/5970879/2256550/d3796bb6-9e08-11e3-815a-09a2d81a6db6.png)
![power3](https://f.cloud.github.com/assets/5970879/2256551/d3906168-9e08-11e3-999d-848f69f24e7b.png)
Although not related to the similarity laws, this behavior can be explained by the fact the electrical motors usually become more efficient for higher powers and that this is not reflected in the power law. Is there a way this can be implemented in the buildings library as well?

Kind regards,

Filip