get_time_bars returning unix timestamp in date_time column

[mdalvi]

Describe the bug
The mlfinlab.data_structures.get_time_bars(**kwargs) is generating unix timestamp flavor for returned date_time column.

To Reproduce

t0 = time.perf_counter()
# 15 minutes chosen for legacy reasons
df_time_bars = ml.data_structures.get_time_bars(df[['date_time', 'price', 'volume']], resolution='MIN', num_units=15, batch_size=1000000, verbose=False)
print(f"Finished ml.data_structures.get_time_bars in {time.perf_counter() - t0} seconds")

Expected behavior
Expected behavior, date_time column should be in yyyy-mm-dd HH:MM:SS.ffff format

Screenshots

Desktop (please complete the following information):

• OS: [e.g. iOS] OS Name Microsoft Windows 11 Home Single Language Version 10.0.22000 Build 22000
• Browser [e.g. chrome, safari] Chrome Version 98.0.4758.102 (Official Build) (64-bit)

Additional context
Python 3.8.12
mlfinlab 1.5.0