Make invoice multispecialised.

Store not so important categories on movements instead of on invoice itself.

BUGFIX. Fix function used to find Slave Instance related with Software Instance that will be destroyed. When the state of Slave Instance was stopped, this object was ignored. Now, all states are considered to destroy

Nothing shall be delivered in this moment. Only top level Simulation Movements
shall be buildable.

During development document can be added to this list.

Invoice can gather movements with many dates.

New step marks implementation requirement for test.

It is *excepted* to have all subscription Sale Packing Lists to be built in
one shot.

In Vifib dates of documents shall follow parent simulation movement dates.

It is required to play on simulaitons in simple way.

Assume 12th periods of full month, but in same month.

Start with current month and go until *end* of this month, then 11 times full
months.

string to vifib master.

This is a partial cherry-pick of 68a50590.