feat/adding function to fuse sequential elements into one

[Dominik-Vogel]

Implemented as function and not as operator, so that additional arguments are possible. Possibly add operator later for default args. Possible abiguity: does plus sequentially add elements or by channel?

function like in doc string:

    joins two elements by appending elem2 to elem1
    and returns a new atomic element:
    |-~-| |~--| |-~-~--|
    |---|+|~--|=|---~--|
    |---| |~--| |---~--|
    If the channels specified in the two elements differ
    one can choose to do zero padding
    |-~-| |~--| |-~-~--|
         +|~--|=|---~--|
    |---|       |------|
    If the channels are disjunct an overlap can be introduced
    |-~-|       |-~--|
          |~--| |-~--|
         +|~--|=|-~--|
    |---|       |----|