Deal with method previous() in DeepTrackNode

[giovannivolpe]

Background Issue

The method previous() in DeepTrackNode seems largely a repeat of current_value() minus no error if the ID is not valid (but still an error if the value is not yet calculated). Furthermore, it seems to be used only in SequentialProperty.

Possible Solution

• Short-term: Move previous() into SequentialProperty (seems to work).

• Long-term: Eliminate previous() and use current_value() in SequentialProperty (it gives some errors though).

• Documentation, tutorials and unit tests for DeepTrackNode need also to be adjusted accordingly.

[giovannivolpe]

From @BenjaminMidtvedt:

The core idea is that for sequential data, when calculating at step N, we store steps 1...,N-1 in a list and optionally provide it to the property value function to calculate step N. This method is important on step 1, to initialize the data list as an empty list (hence returning [] if there's no previous value)

This only makes sense as a part of a sequential property, and should ideally be moved there.