get_band_of_wavelength: Get The Measurements Made In A Chosen Wavelength
In pieterbeck/CanHeMonR: Tools for canopy health monitoring through image analysis

Extract from a dataframe where columns are measurements in individual wavelengths, and named following Quantalab's convention, the data that are closest to a chosen wavelegth.

1 2	get_band_of_wavelength(spec_df, wavelengths_in_nm = NULL, wavelength_range = NULL, band_txt = "X", splitter = "[X.]", i = 2)

`spec_df`	Dataframe or RasterBrick of spectral measurements, with a column per band/spectral wavelength, and columns (layers if spec_df is RasterBrick) named following Quantalab's convention.
`wavelengths_in_nm`	Numeric vector. The wavelength(s) of interest, in nm. Defualt is NULL
`wavelength_range`	Numeric vector of length 2. Instead of requesting observations (columns or rasterlayers) for specific wavelengths (through wavelengths_in_nm ), one can requests all observations falling in a specific wavelength range to be returned in a given range. Default is NULL
`band_txt`	Character. A piece of text that only occurs in columns/layers that have radiance/reflectance values. Default is "X"
`splitter`	character. The character that can be used in strpslit to isolate the wavelength value. Default is "[X.]"
`i`	The position of the wavelength value after the stringsplit operation.