Updated udp_receive_marker.py to receive byte input instead of JSON

[bchung2017]

This pull request concerns the sample Python scripts mentioned as the sample UDP code for OpenBCI/Documentation#188

Previously, udp_receive_marker.py expected incoming UDP packets to be in JSON format. However, udp_send_marker.py (and the OpenBCI GUI's Java implementation) sends raw 4-byte data instead. This mismatch caused decoding errors when receiving markers, as shown below:

(venv) bchung2017@LAPTOP-N4UVVLMN:~/openbci$ python3 -m udp_receive_marker --ip 127.0.0.1 --port 12345 --option print
--------------------
-- UDP LISTENER --
--------------------
IP: 127.0.0.1
PORT: 12345
--------------------
print option selected
Listening...
'utf-8' codec can't decode byte 0x8b in position 3: invalid start byte
'utf-8' codec can't decode byte 0x80 in position 2: invalid start byte
'utf-8' codec can't decode byte 0xe0 in position 1: invalid continuation byte
<continued>

The script has been updated to properly handle raw byte data, ensuring compatibility with udp_send_marker.py and OpenBCI GUI. After the fix, the sample script now correctly receives and prints markers:

(venv) bchung2017@LAPTOP-N4UVVLMN:~/openbci$ python3 -m udp_receive_marker --ip 127.0.0.1 --port 12345 --option print
--------------------
-- UDP LISTENER --
--------------------
IP: 127.0.0.1
PORT: 12345
--------------------
print option selected
Listening for UDP packets...
Received Marker: 2.506424330049714
Received Marker: 1.5585284066901215
Received Marker: 2.8304827346407384
Received Marker: 1.0063118242670144