Allow users to create an SSM channel for Tx Labels

[jtcalvert]

PR #82 removed the ability to create special channels (SDI, SSM, Parity) for Tx labels. However, there are use cases where users may want to manually manipulate the SSM and SDI bits in order to test certain failure conditions.

Currently, you can create custom parameters that use bits 8-9 (SDI) or 29-30 (SSM) in your Parameters XML file, which will let you manually manipulate those bits for Tx Labels:

However, it would be a better customer experience if they were able to create the SSM and SDI channels using the same createSDIChannel and createSSMChannel tags in the Parameters XML file as used for Rx channels.

FYI @jkreienh