Helper functions

[Daniel Russel <drussel@gmail.com>]

I have various helper functions that are probably generally useful. I  
am ambivalent about what to do with them.
One set is utilities for getting and setting attributes which look like
/**
	Set the value of the string attribute name to val, whether or not it  
was already there.
*/
bool set_string(ModelData *m,
                  Particle *p,
                  const String& name,
                  std::string val)
and
/**
	Get the string attribute name or return default_value if the  
particle does not have any such attribute.
*/
const String& get_string(ModelData *m,
                  Particle *p,
                  const String& name,
                  const String& default_value=String())
 Obviously there are int and float versions.

Should such things go in to imp proper? On one hand they are  
extremely useful. On the other it is increasing the size of the  
interface and while it makes it more convenient, it doesn't add any  
power.


Next I have some helpers for manipulating hierarchy and bond  
particles (getting the number of children, getting specific children  
etc). These should go somewhere to establish norms for hierarchies.