[Showcase_OnlineSensorBased] Errors and warnings

[jorasinghr]

When running dataset.calc_daily_average an error message is shown:
TypeError: float() argument must be a string or a number, not 'Timestamp'.

There are some minor errors in the same file;
when running dataset.fill_missing_model a warning is shown, FutureWarning: 'argmin' is deprecated, use 'idxmin' instead. The behavior of 'argmin'.
Another warning is shown when running dataset.get_correlation, ImportWarning: can't resolve package from __spec__ or __package__, falling back on __name__ and __path__.