Save numbers in {:g} format not {!r} format in prettify function.

The prettify function, originally part of cantherm (3f7c71baa2c07d6bd21aadde853a487dddf503b7)
is used to save entries in the database. It turns all numbers into repr(number),
even if you had previously tried to save them as something else.
Now, it uses {:g} formatting, so instead of 
 A = (3600000000000000.0, 'cm^3/(mol*s)')
we now get
 A = (3.6e+15, 'cm^3/(mol*s)')

This will have broad-ranging impacts on file output - I hope they are all positive.
If not, we will have to make it configurable on a case-by-case basis somehow.