Added plotting of heatmap with logarithmic X axis

Also updated default film settings.
Added reusing intermediate text data for film

Added --no-clean option for film, can use data from TEMP_FILM to replot heatmap
frames.
Added appropriate tics for logarithmic heatmap plot
Implemented support for pyxplot as plotting engine

There is now choice between gnuplot and pyxplot for plotting heatmaps. Data
file format accepted by gnuplot changed to 3-column ASCII.
Added option to plot selected iterations
Implemented metadata writing into film