Factors

A package handling the retrieval of custom factors has been added. The only custom factors we support at the moment are the hepspec values and this is the only information we have on mongodb at the moment. 

Here is how the output of the respectful call will look like:

```xml
<root>
 <Factor site="AEGIS01-IPB-SCL" weight="13516"></Factor>
 <Factor site="AEGIS02-RCUB" weight="346"></Factor>
 <Factor site="AEGIS03-ELEF-LEDA" weight="614"></Factor>
 <Factor site="AEGIS04-KG" weight="1229"></Factor>
 <Factor site="AEGIS09-FTN-KM" weight="28"></Factor>
 <Factor site="AEGIS11-MISANU" weight="563"></Factor>
 <Factor site="AM-02-SEUA" weight="440"></Factor>
 <Factor site="AM-04-YERPHI" weight="440"></Factor>
 <Factor site="ARAGRID-CIENCIAS" weight="7411"></Factor>
 <Factor site="AREA-BO" weight="675"></Factor>
 <Factor site="ARNES" weight="37671"></Factor>
 .
 .
 .
</root>
```