Add API for IQ channel

[mjhaye]

This is how I have done it now for lls. Still based on the IQ_channel_constructor to keep the implementation identical, but maybe a new/double implementation is preferred (to avoid circular dependencies).

Also the name define_iq_channel looks a lot like define_IQ_channel but I could not think of something better and eventually the latter should be removed, I think.

Also added py.typed so that the typehints that exist at least are available to external users.

[sldesnoo-Delft]

Thanks for the contribution. I'll copy the API you propose.
I'm going to change the implementation to make it a lot cleaner.
IQ_channel_constructor.init() now calls pulse_lib.define_IQ_channel().
When pulse_lib_define_iq_channel() calls IQ_channel_constructor.init() calling pulse_lib.define_IQ_channel() then at least I get a bit confused.

[sldesnoo-Delft]

Committed on dev branch:
6ca2772

To be tested with hardware. Release today or tomorrow.

[sldesnoo-Delft]

Changed API to:
def define_iq_channel(
self, name: str, i_name: str = "", q_name: str = "",
i_neg_name: str = "", q_neg_name: str = "",
marker_name: str = ""
) -> None:
'''
Defines a new IQ pair for IQ modulated MW pulse generation via qubit channels.

    For balanced IQ signals two additional channels may be specified for the
    negative image of I and Q signal.

    Args:
        i_name: awg channel name of the I component (+)
        q_name: awg channel name of the Q component (+)
        i_neg_name: [optional] awg channel name of the I component (-)
        q_neg_name: [optional] awg channel name of the Q component (-)
        marker_name: [optional] marker to use for pulse modulation.
    '''