`df.std()` does not infer types of generated columns

[Jolanrensen]

df.std() does not infer types of generated columns.

An example can be found in this notebook:
https://gist.github.com/Jolanrensen/c078d45c78ff7057598b40cb9c40b368

The standard deviation of Int columns results in a Double, but DataFrame assumes the type of columns stays the same after the operation.

Patching this fixes 7/15 tests in #713