True when expr is an integer or rational power of 2, otherwise False.
abjad> for n in range(10):
... print n, mathtools.is_power_of_two(n)
...
0 True
1 True
2 True
3 False
4 True
5 False
6 False
7 False
8 True
9 False
abjad> mathtools.is_power_of_two(-4)
False
abjad> mathtools.is_power_of_two('foo')
False