Fixed line colors cycle + plot_valid + covariance matrix shape and slicing

[Didou09]

Main changes:

• plot_valid() fixed (in some cases refY was a tuple, but it a str was expected)
• covariance matrix ref and shape was not generic enough
  • (nx_free, nx_free) was appended at the end
  • now inserted where appropriate to preserve shape of data
  • updated slicing accordingly
• line color cycling now more robust using modulo

Issues:

Fixes, in devel, issues:

• color_cycle error in coll.plot_spectral_fit() if fitting >10 lines #81
• Tutorial not fully operational #82