New vignette and bug fix

• Bug fix: test_spectra() now returns trained model correctly when only one pretreatment is specified.
• Change the gap-segment derivative pretreatment to retain compatibility with prospectr. In the upcoming version of prospectr, the gapDer function only accepts odd values for the segment argument in order to properly compute the convolution filter.
• Default plot title for plot_spectra() is now NULL (no title) if detect.outliers is set to FALSE.
• Column names in output list item $summary.model.performance from test_spectra() now include underscores rather than periods for easier parsing.
• Update website
• New vignette: vignette("waves")