ENH: pivot_annual with a better exception message
Without the extended message the user hardly understand why no integers are allowed.

Actually, I do not even understand it by now.

Can we not use _values.fill(np.nan)_ with integers?

I pushed a change to allow series with integer dtypes in pivot_annual
You cannot use values.fill(np.nan) with integers because np.nan is a float.
The new change ignores the input dtype and returns float dtypes 
