set_proxy
Description:
Sets the proxy for the asset.
If proxy is among the existing proxies of the asset (see 
      list_proxies) it will be moved to become the default proxy. 
      Otherwise, if proxy is not null, it will be added to the list of proxies, as the new default. The
       previous default proxy will become 'next in line' for if the new one is removed, and so on. As such, this will **not** actually remove the 
      previous default proxy (use unproxy for that).
Note that an asset can only act as a proxy for one other asset.
As a special case, if proxy is null, then this method will actually remove **all** proxies from 
      the asset.
Parameters:
| this | 
           The Asset to proxy  | 
      
| proxy | 
           A new default proxy for this  | 
      
Returns:
| 
           true if   |