Allocate Buffer Size for Read/Write String Status Properties

[charitylxy]

• [x] This contribution adheres to CONTRIBUTING.md.

What does this Pull Request accomplish?

This PR aims to address issue mentioned in https://github.com/ni/nidaqmx-python/issues/189:

• Currently, buffer size is set to 0 initially here and will return the actual buffer size needed after calling xxx_chans for the first time. Calling xxx_chans for the second time (with the actual buffer size) will return error -201194 due to AB#2393824.
• This PR is a temporary workaround to get the buffer size at xxx_chans property and passing the buffer size to get_read_attribute_string or get_write_attribute_string at _library_interpreter so that xxx_chans doesn't need to be called more than once.

Why should this Pull Request be merged?

In this PR to address https://github.com/ni/nidaqmx-python/issues/189, following changes have been made:

• Buffer size will be determined based on number of characters in task.channel_names.
• Buffer size will be allocated in xxx_chans property and passing it to get_read_attribute_string or get_write_attribute_string as size_hint.
• size_hint=0 by default if buffer size is not assigned (for non xxx_chans method/property).

What testing has been done?

Manually tested overtemperature_chans with overtemp real device TS-15200.

import pprint
import nidaqmx

with nidaqmx.Task() as task:
    # Channel Settings
    voltage_setpoint = 0
    current_setpoint = 0.03
    output_enable = True

    task.ai_channels.add_ai_power_chan(
        "TS1Mod1/power", voltage_setpoint, current_setpoint, output_enable
    )
    task.start()

    if task.in_stream.overtemperature_chans_exist:
        print(f"overtemperature chans: {task.in_stream.overtemperature_chans}")

    task.stop()

Results: overtemperature chans: ['TS1Mod1/power']