Rename 'vanderMonde()'

[cbeleites]

This is old issue https://github.com/cbeleites/hyperSpec/issues/339

[cbeleites]

vanderMonde() should be renamed - if only because I misspelled the name (thinking it was Dutch van der Monde when it is French Vandermonde).

I propose to rename it into poly(), make that generic and extend the functionality to provide orthogonal polynomials:

• vanderMonde() calculates so-called raw polynomials, either directly on the wl() of a hyperSpec object or on some rescaled version of it.
• stats::poly() calculates by default orthogonal polynomials, but can be used to calculate raw polynomials as well (via raw = TRUE)
• The most important use of vanderMonde() I have is setting up polynomial terms for EMSC, and for that orthogonal polynomials are better than raw polynomials or polynomials with the wavelength scaled to [0, 1] or [-1, 1].
• The one thing I'd rather have different from the default behaviour of poly() is that I'd like to include an offset (degree 0) term.