DataFrame indexing and formatting improvements and creation speed-up

• Added new parameters to pickle and dict creation functions for specifying ID or cycle mode column headings
• Made int, float and datetime.datetime conversion from strings part of pickle/dict creation
• Added column metadata to pickled dicts and dicts, pairing them and records dict within overall dict
• Made DataFrames retain original column headings from text files across all index and data columns
• Added cols_to_ignore, which takes either string or integer arguments
• Added df_select_datetime_range function for slicing DataFrames
• Added cycle mode to MultiIndex in cycling records and DataFrames