Better xtick labels in concentration plots

Nicolas Mounetrequested to merge
develop/hours2time_in_concentration_plot into master

Labels for the x-axis are with the proper time format hh:mm (e.g. 14:30 or 8:36). Some redundant definitions of the function hours2time are also avoided.

concentration_curve1

concentration_curve2

Solves #136 (closed)

Edited by Nicolas Mounet

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