Add multi-function synchronization examples

[WayneDroid]

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

What does this Pull Request accomplish?

• add multi-function synchronization examples
• add analog acquisition on digital reference example (miss out from last PR)
• minor update on callback function for every n samples event example

Why should this Pull Request be merged?

Synchronization examples are required to be released with version 1.0 as specified in the feature scope.

What testing has been done?

run poetry run pytest tests -q - pass

run examples on physical devices PXIe-6363 - okay

C:\dev\repo2\nidaqmx-python>poetry run python examples\multi-functions-synchronization\cont_ai_di_acq.py
Acquiring samples continuously. Press Enter to stop.

Read:   AI      DI      Total:  AI      DI
        1000    1000            4000    4000

C:\dev\repo2\nidaqmx-python>poetry run python examples\multi-functions-synchronization\ai_ao_sync.py    
Actual sampling rate: 1000 S/s
Acquiring samples continuously. Press Enter to stop.
Acquired data: 1000 samples. Total 5000.

Acquired 5000 total samples.

C:\dev\repo2\nidaqmx-python>poetry run python examples\analog_in\voltage_acq_int_clk_dig_ref.py     
Acquired data: [0.802982, 0.803466, 0.803144, 0.803144, 0.802982, 0.802177, 0.801533, 0.800889, 0.800083, 0.799922, 0.799439, 0.798956, 0.799439, 0.799278, 0.799761, 0.800244, 0.800728, 0.801694, 0.802016, 0.802660, 0.802982, 0.803144, 0.803144, 0.802982, 0.802660, 0.802338, 0.801694, 0.800889, 0.800405, 0.799761, 0.799600, 0.799117, 0.799117, 0.799600, 0.799761, 0.800083, 0.800889, 0.801533, 0.801855, 0.802499, 0.802982, 0.803305, 0.803144, 0.802821, 0.802499, 0.802499, 0.801533, 0.800889, 0.800728, 0.799922, 0.799761, 0.799117, 0.799278, 0.799278, 0.799761, 0.800244, 0.801050, 0.801694, 0.801855, 0.802660, 0.803144, 0.803466, 0.803466, 0.803144, 0.802821, 0.802338, 0.801694, 0.800889, 0.800567, 0.799922, 0.799439, 0.799117, 0.799278, 0.799439, 0.799439, 0.800567, 0.801050, 0.801372, 0.802016, 0.802660, 0.803144, 0.803466, 0.803144, 0.803305, 0.802982, 0.802338, 0.801694, 0.800889, 0.800405, 0.799922, 0.799439, 0.799439, 0.799439, 0.799278, 0.799761, 0.800405, 0.800889, 0.801533, 0.802177, 0.802499]