lisa.datautils: add df_convert_to_nullable()
lisa.datautils: add df_convert_to_nullable()

Add df_convert_to_nullable(), allowing to convert the
columns of a dataframe to their equivalent nullable datatype.

Reference:
https://pandas.pydata.org/pandas-docs/version/1.0.0/user_guide/integer_na.html

Signed-off-by: Pierre Gondois <b3d58e8216be8f0fcc0dc2a3d8b3f0eae8029a28@arm.com>

ARM-software/lisa,credp/lisa,ARM-software/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa,credp/lisa,credp/lisa