Corrected bug where the tree started even for very low fields.
Allow to change any parameter when the lowest streamer reaches an
altitude threshold_z.  This is done by introducing th optional section
[lower] in the .ini file.
To implement this I took rid of the single_branching_z parameter, whose
functionality can be easily implemented with the new, more general behavior.
However, note that this implies a break of backwards compatibility: some
older input files may not work anymore.
Improvements in plot.py to allow parallel plotting with multiprocessing.
Improvements and bug corrections to plot.py
Corrected a bug where the field for the effective ionization was not calculated
correctly.
Provide an error message under some conditions of misconstructed input file.