Call the _is_ready() function before sending any command

[johndoknjas]

In order to prevent lagging. E.g., one could call a function many times, where this function sends an expensive command to Stockfish. The function calls would be relatively quick, but they would finish before Stockfish can process all the commands. This will make subsequent commands lag.

So calling _is_ready() before sending a command ensures Stockfish doesn't get bogged down working through multiple commands. And _is_ready() itself is quite fast (hundredths of a millisecond on my computer). _is_ready() can also be sent after certain expensive commands, assuming that they don't output to stdout (since _is_ready will clear this).