Refactor hk98.py module to improve readability and documentation

[taruma]

Ringkasan Pull Request

refactor hk98

Perbaikan/Fitur

close #248

Penggunaan

This module provides functions for calculating summary statistics on datasets.

Functions:
- summary_station(dataset, column, ufunc, ufunc_col, n_days='MS'): 
    Calculate summary statistics for a given dataset and column.
- summary_all(dataset, ufunc, ufunc_col, columns=None, n_days='MS', verbose=False): 
    Calculate summary statistics for multiple columns in a dataset.

For more details, refer to the manual: 
https://gist.github.com/taruma/aca7f90c8fbb0034587809883d0d9e92