Check for any/proper DatetimeIndex in observations

[jrmontag]

The pandas wrapper in decompose() keeps track of the observation DatetimeIndex so it can be used properly in the forecast() method.

There are two potential issues here:

• while the decomposition seems to work fine without continuous data e.g. no missing data, it does require a DatetimeIndex
• though the decomposition would work with missing data, the forecast will fail awkwardly if there is a bad DatetimeIndex

A couple of possibilities:

• the decompose method could raise an exception if the input is missing a DatetimeIndex
• the forecast method could catch exceptions and raise this as a possibly helpful message