Fix memory leak in SerialCom

The temporary char array pointer used to read from the file descriptor before converting to a string was never deleted. This fixes the memory leak (and speeds up the code) by making the temporary array a member of SerialCom and not a pointer.

Also updated the description of SerialCom::read(std::string&) to reflect how it is being used in all of the code. The maximum read length is not taken from the buffer string, but was always taken to be MAX_READ.