change behavior of SearchConfig

 - rename parameters to the following:
    - table (instead of select)
    - default_sort (instead of order_by)
 - validate parameters on initialization
 - remove sort_columns and search_columns (no longer used by RESTAPI)
 - rename columns_for_filtering to columns_for_searching
 - rename order_by_column to column_for_sorting
refactor SearchSystem

 - validate all parameters only once
 - use new parameter names and methods in SearchConfig
 - only pass needed parameters to underlying methods
rename parameters in search config