incremental: Enabling more colors while Plotting multiple instruments

When plotting multiple instruments, a wider range of colors might be needed. To that end, the SubPlot class has been altered so that the following can be done

plt._StrategyPlotter__portfolioSubplot._Subplot__updateColor(9) -> will enable 9 different colors used as colormap for a given plot (using default mapping='Accent'
plt._StrategyPlotter__portfolioSubplot._Subplot__updateColor(9, 'terrain') -> same as above but 9 colors will be determined from the 'terrain' map
plt._StrategyPlotter__portfolioSubplot._Subplot__resetColor() -> will bring back default pyalgotrade color map (=5 colors)