fix `as.incfreq()` warnings, clarify documentation and add tests

[stragu]

as.incfreq() would throw a warning when the presence-absence data was only 0s or only 1s.

For example, try the following in the current release:

test_df0 <- data.frame(col1 = rep.int(0, 10))
library(iNEXT)
as.incfreq(test_df0)

This PR fixes the following:

• Make it so warning is not shown when only either 1 or 0 is present
• Clarify warnings and documentation
• Use warning() instead of warnings() for the vector warning (assumed to be a typo)
• Add tests for as.infreq()

I am not particularly familiar with the package, so wanted to also ask:

• Is the current behaviour for NAs correct? (As in, keeps the NA, and returns NAs as row sums)
• What should happen if there are negative values in the matrix or dataframe? Currently, they are not convert to 1s in line 374.