core.py: Enable correct exporting of char data

[junwoo091400]

Description

As the data type for saving the char in pyulog was np.int8, the topics that had fields with characters weren't able to get encoded (with error: pyulog struct.error: char format requires a bytes object of length 1).

Therefore, this enables manual conversion of np.int8 object into bytes() with single character, so that struct.pack command works as expected.

Notable example of where this is needed is the "transponder_report" topic (included in PX4 v1.14)'s callsign[] character array. Before this fix, the "write_ulog()" function would fail as the struct.pack couldn't correctly encode the characters stored in int8 type

Testing

Before

Following simple script importing & exporting a ulog file from a PX4 v1.14 vehicle didn't work before (example log downloadable here):

from pathlib import Path
import numpy as np
from pyulog import ULog

ulog_path = '' # Set to the path of the log

TOPIC_NAME = 'transponder_report'
FIELD_NAME = 'callsign[0]'
ulog = ULog(str(ulog_path.absolute()))
dataset = ulog.get_dataset(TOPIC_NAME)
name = dataset.name
field_data = dataset.field_data
timestamp_idx = dataset.timestamp_idx
data = dataset.data
for field in field_data:
    if field == FIELD_NAME:
        print('Field name: {}, type: {}'.format(field.field_name, field.type_str))
field_values = data[FIELD_NAME]
print('Field values:', field_values, 'type:', type(field_values[0]))

output_file_name = ulog_path.name + '_test' + ulog_path.suffix
print('Writing ulog', output_file_name)
ulog.write_ulog(Path.joinpath(ulog_path.parent, output_file_name))

With the following error:

Field values: [0] type: <class 'numpy.int8'>
Writing ulog power_on_without_rc.ulg_test.ulg
Traceback (most recent call last):
  File "scripts\another.py", line 23, in <module>
    ulog.write_ulog(Path.joinpath(ulog_path.parent, output_file_name))
  File "venv\Lib\site-packages\pyulog-1.1.0-py3.11.egg\pyulog\core.py", line 253, in write_ulog
    self._write_data_section(ulog_file)
  File "venv\Lib\site-packages\pyulog-1.1.0-py3.11.egg\pyulog\core.py", line 401, in _write_data_section       
    items = self._make_data_items()
            ^^^^^^^^^^^^^^^^^^^^^^^
  File "venv\Lib\site-packages\pyulog-1.1.0-py3.11.egg\pyulog\core.py", line 429, in _make_data_items
    data.extend(struct.pack('<' + field_encoding, field_data))
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
struct.error: char format requires a bytes object of length 1

After

After the change the error doesn't pop up and exporting the ULog file works as expected.

[bkueng]

Thanks, can you fix the CI error? Otherwise looks good.