Review and (implement) SynchroMode

Currently synchroMode is passed as a parameter to PLC::connect but is never used. While on each (re)connect the registers are read from the PLC.

SilecsPLC.h:312:

* \param synchroMode = NO_SYNCHRO: connect() method does not synchronize any retentive Registers at connection time.
* synchroMode = MASTER_SYNCHRO: connect() method downloads all the Master Registers from the PLC memory and updates local Register values at connection time.
* synchroMode = SLAVE_SYNCHRO: connect() method transmits all the local Slave Registers values to the PLC at connection time.
* synchroMode = FULL_SYNCHRO: connect() method executes MASTER and SLAVE Registers synchronizationat connection time (see above).

TODO:

• Consider if the synchroMode should be properly implemented or removed entirely.