What is the point of this function?  It doesn't seem to ever be called.  When we need a new realization of the power (in buildGriddedShears), we just make a new PowerSpectrumRealizer instance.  

Of course, it doesn't hurt to have it, so it's fine to leave this as is.  I'm just curious as to why it was put in.  (Or why init was broken up into set_size and set_power for that matter, which seems to have been specifically to enable this function.)