Add to fit_peaks() support for additional methods

[aphalo]

Currently the only method supported is spline, which is usually o.k. for estimating the wavelength location but not the height of a peak. Fitting Gauss, Lorentz and Voigt functions could be implemented as these are frequently used, at least when peaks are on a low baseline

See files in folder inst-not for background information. These would need to use nls() and some kind of self starting functions.

See also function fit_peaks() in package 'chromatographR'.

[aphalo]

Examples from Alan Morison