Globally replace the word source in the code wherever source processes is meant

[Abzinger]

The user facing side of the code talks about sources wherever the source random processes feeding information into are target are meant.

Inside some classes, the word "sources" is used to specify a single random variable, i.e. the combination of the index of a random process and a time lag.

This can be pretty confusing for newcomers trying to contribute.

[pwollstadt]

Replace variable name source by either source_process whenever the whole process is meant or by source_variable whenever a single random variable in the past of a process is meant (a process is a collection of random variables indexed by a time index t).