Option to `element,get_value` etc to return formatted string with units

[aawdls]

It would be nice if we could get the unit name together with a value without having to go and get the unit conv object.

A possible implementation would be to add an optional argument e.g. formatted to e.g. element.get_value which would make it return a string with a few digits of precision and the units.

People may want then to specify the precision...

[willrogers]

#98 added the units but they're not that useful yet because of this.

[T-Nicholls]

This is a good idea. However, I would suggest a new method for returning formatted data like this for three reasons:

• I think there are already lots of optional arguments to get_value, so I am cautious about adding more.
• This feels like something ripe for future expansion, i.e. also returning the PV name, conversion limits ect., which would not be as feasible if it were embedded inside get_value.
• Calling a method that then calls get_value and get_unitconv and then formats and returns the value and the appropriate units feels more intuitive to me than tacking it onto the end of get_value.