Fix post-checkout git hook script.
Add PlotSample class, minor plotting fixes.

Add PlotSample class for plotting of samples. The class adds additional
fields 'system_time_c', which corrects 'system_time' for overflow, and
'real_time', which converts 'system_time_c' into seconds. Also adds
method set_default_x() and plot_d() for setting and plotting with a
default x axis.
Fixes in the following files:
    - message_np.py: Fix typo, print last valid system time if a
        DecodeError occurs.
    - plot_data.py: Change PlotData to new-style class, add members to
        store additional data, don't overwrite builtin 'type', change
        order of nested fields in plots, fix color cycling so prevent
        the first and last plot from being similar colors, add
        method get_field() which can return a base field or an
        additional field, plot() now returns the figure.