Allow new `hyperSpec` object without wavelengths

[GegznaV]

This is cbeleites/hyperSpec#296

---

Suggested:

new("hyperSpec", data = data.frame(c = 1:5))

this works:

new("hyperSpec",
     wavelength = numeric(0),
     spc = matrix(NA_real_, ncol = 0, nrow = 5), 
     data = data.frame(c = 1:5)
)

[GegznaV]

Currently, the official solution to this issue in hyperSpec is to use as.hyperSpec():

as.hyperSpec(data.frame(c = 1:5))

as.hyperSpec(as.matrix(data.frame(c = 1:5)))

as.hyperSpec(matrix(1:5))