Rename, comment and modernise graph tutorials

[emachiamu]

<> graph.C : Rename to gr001_basic.C, add comments, create two sections for uploading data from a known array and from a text file.
<> graphApply.C: Divide in two tutorials: gr004_er_asym.C (explaying TGraphAsymmError) and gr005_apply.C (focusing on the Apply() method).
<> multigraph.C: Rename to gr007_multigraph.C and add comments.
<> gmultierrors.C: Rename to gr008_multierrors.C and add comments.
<> graphShade.C: Rename to gr101_shade_area.C and add comments.
<> graphreverse.C: Divide in two tutorial: gr102_reverse_axis (reverse method applied to TGraph) and gr102_err_reverse_axis (reverse method applied to TGraphError, TGraphAsymmError and TGraphBentError).
<> gr110_logscale.C: Create a new log scale tutorial.
<> gr111_legend.C: Create a new legend tutorial.