Implement remaining string format codes for floats #1656

>>> '{:g}'.format(10.0)
'10'
>>> '{:.1g}'.format(10.0)
'1e+01'
>>> '{:e}'.format(10.0)
'1.000000e+01'
>>> '{:.1e}'.format(10.0)
'1.0e+01'
>>> '{:n}'.format(999999.1234)
'999999'
>>> '{:n}'.format(1000000.1234)
'1e+06'
>>> '{:.8n}'.format(1000000.1234)
'1000000.1'
>>> '{:.11n}'.format(1000000.1234)
'1000000.1234'
>>> '{:%}'.format(10.0)
'1000.000000%'

>>>>> '{:g}'.format(10.0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Format code 'g' for object of type 'float' not implemented yet
>>>>> '{:G}'.format(10.0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Format code 'G' for object of type 'float' not implemented yet
>>>>> '{:e}'.format(10.0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Format code 'e' for object of type 'float' not implemented yet
>>>>> '{:E}'.format(10.0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Format code 'E' for object of type 'float' not implemented yet
>>>>> '{:n}'.format(10.0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Format code 'n' for object of type 'float' not implemented yet
>>>>> '{:%}'.format(10.0)
'10.0'