Reads a given number of registers from the slave.
Note that the number of registers returned (i.e. array length) will be according to the number received in the slave response.
@param unitid the slave unit id.
@param ref the offset of the register to start reading from.
@param count the number of registers to be read.
@return a
Register[] holding the received registers.
@throws ModbusException if an I/O error, a slave exception ora transaction error occurs.