Pandas dataframe export

[lasch]

Introducing a new export format based on pandas dataframes.

Adding a new output format (-f pddf). When using from cmdline, the output is a text file with a table containing the exported events (only completeevents; ph="X"). When using as a library, using Acelyzer.get_output_data() returns a pandas dataframe object for subsequent processing.

Initial number of columns is maybe limited. A later edition may allow customization of the table content via this class member:

            self.data_map = {
                "ts": ("Timestamp", 0.0),
                "dur": ("Duration", 0.0),
                "cat": ("Category", "other"),
                "name": ("Event Name", "NoName"),
                "args.pt_active": ("PT_Active", 0.0)}

Readme has been updated to include description and example of the feature.