Add channel filtering/ordering to pulse Schedule -> Signal converter

[DanPuzzuoli]

What is the expected behavior?

Connected to #53.

Update the converter to include optional argument channels, specifying a list of channels as strings e.g. ['D0', 'D2', 'U0'], which modifies the behaviour of the function to:

• Filter out channels not included in the list.
• Return signals in a list given by the ordering in channels.

[DanPuzzuoli]

We also need to add test cases for this function.