v0.5.7 SQL Escaping; pytest; defaults in to_donpa(), to_pandas(); improved from_pdf()

v0.5.7
Change SQL identifier escaping from character escaping to quoting.
Note, this is a breaking change if prior SQL tables are encountered.
Move to using pytest. Removed some sys.path.append() statements as a result.
Improve default handling in to_donpa() to allow None to be the default by using a sentinel _MISSING
Improved .to_list() default handling to allow None as the default, by using sentinel _MISSING.
Improved .to_pandas_df() default handling
Added option use_donpa to .to_pandas_df() to improve convertion to pandas df by first converting columns to numpy vectors.
Added tests for .to_list() including various options.
Added .replace_in_columns() to Replace all values in find_values with replacement for the specified columns.
Added .apply_colwise() Apply a function to each row and store the result in target_col
Improved .to_pandas_df()
to provide an option to convert first to donpa (dict of numpy array)
to handle conversion of '' or None to a default such as NA or 0.
improved tests.
Improved from_pdf() to support complex conversion scenarios.