Allow user to specify a delay

The user may specify a delay between PACE nonce requests. Using no delay between subsequent issues of the USB command may lead to errors in the USB communication.
Until test results show that the delay can be fixed or left out completely the user has to test what value is safe to use.

This fixes errors "Read only 40 instead of requested 64 bytes!" (if the correct delay is used, e.g. 3 seconds for 16 byte nonces).