Change name of plot_mf_phase_plot to plot_mf_phase

- Remove code that handled a data file
Change axes labels in function plot_mf_phase()
Add some functionalty and fix a bug

- Add function plot_mf_only_preys. This function plots the time series
  corresponding to a model that considers intraspecific competition and
  prey reproduction only.

- Add function plot_mf_seck. This function plots a mean field model that
  uses the equations proposed by Dr. Seck. Only intraspecific
  competition and prey reproduction terms are considered.

- Import log from scipy.

- Fix a bug that where a non normalized plot is shown in the function
  plot_time_series_normalized() if a time range is given.