Add support for unequal series lengths for MSSA. Along the lines:

1. Introduce new kind of "elementary series": series list. Overload the arithmetic ops on it
2. Streamline the hanlding of attributes
3. Get rid of .slength
4. Make sure the common methods do no decisions about e.g. contents
5. Cleanups here and there
Implement fixup for .apply.attributes
Add k-forecast