Use more consistent signature for methods of as.hyperSpec()

[GegznaV]

Currently, each method of as.hyperSpec() uses its own order, default values, and number of arguments:

.as.hyperSpec.matrix <- function(X, wl = NULL, ...)

.as.hyperSpec.data.frame <- function(X, spc = NULL, wl = NULL, labels = attr(X, "labels"), ...)

.as.hyperSpec.hyperSpec <- function(X)

This makes it more difficult to use in other functions as each case should be addressed separately. My suggestions would be to:

• use NULL for the default values in the signature and perform calculations or real initial values inside the methods;
• use consistent order of arguments;
• rename argument wl into the name that will be suggested in cbeleites/hyperSpec#90.

May also be related:

• https://github.com/cbeleites/hyperSpec/issues/144#issuecomment-750338337

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This issue originally was:

• https://github.com/cbeleites/hyperSpec/issues/297