A convenience method which coerces the specified value to a float.
If the value is an invalid literal for float, a ValueError will be raised.
>>> a = ConfigObj()
>>> a['a'] = 'fish'
>>> a.as_float('a')
Traceback (most recent call last):
ValueError: invalid literal for float(): fish
>>> a['b'] = '1'
>>> a.as_float('b')
1.0
>>> a['b'] = '3.2'
>>> a.as_float('b')
3.2...