Added system in tdma super class for sending modem nmea messages in tdma active slot

Notes:

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• The manual_transmit_queue system is for sending nmea messages to the modem that make the modem transmit. There is only one topic logically compatible right now and it's the nmea_to_modem in the modem namespace tdma is currently servicing.
• in the future there may be other topics or ways of using this manual transmit system. For now, it's just a simple interface for nmea commands that you want to send in band with tdma

New Features:

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• Added system in tdma super class (manual_transmit_queue) for sending modem nmea messages manually to be queued by tdma and sent in active slot (in place of next packet)
• new rosparam ~maxlen_manual_queue default: 1
• new rostopic tdma/manual_transmit_status type String is published to when a message is queued and when message is transmitted (showing contents of queue (useful for when maxlen is >1 or to make sure a later published message is the only one in the queue) diagnostic_msgs/DiagnosticStatus. It shows the contents of all manual transmit queues on insert and on transmit
• User publishes to nmea_to_modem topic inside of tdma/ namespace and tdma will hold up to ~maxlen_manual_queue message(s) and publish them (one at a time) instead of the next outgoing packet during our active_slot.
• If using a value > 1 for maxlen_manual_queue, tdma will work through the manual_transmit_queue until it's empty. It will treat each nmea sentence like a packet with the same packet_length_seconds and guard_time_seconds. It will only transmit one message from this queue at a time so large values for maxlen_manual_queue are not recommended as tdma needs to work through the whole queue before getting back to packets.

Example:

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1. user publishes to $CCACM,1,1,1,CCCFG;pwramp.txlevel;0 to /modem0/tdma/nmea_to_modem
2. $CCACM,1,1,1,CCCFG;pwramp.txlevel;0 is now queued to transmit next
3. when tdma_status.we_are_active and ready to send a packet, we instead publish $CCACM,1,1,1,CCCFG;pwramp.txlevel;0 to /modem0/nmea_to_modem which in this case will make the modem transmit this CCACM.