Unify utils features by theme

Base on comments on #125 (closed)

There are functions for manipulating and operating over pandas dataframes, others to generate SQL code for Cloudera Impala or other similar databases and some functions for data exploration. I would unify all functions that operate over dataframes to a pandas_utils.py, then the sql ones to sql_utils.py and finally remove some functions and leave others in utils.py.

The idea is to unify utils features by theme like:

• pandas_utils when operate / manipulate pandas dataframe
• sql_utils when operate / manipulate SQL queries
• ...

Related to: #129 (closed)

Very very very very old but related: #3 (closed)