throw warning from R when attempting to process S-band data in single-pol C-band mode

[adokter]

Currently only the vol2bird stdout throws a warning when processing S-band single-pol data with mistnet=FALSE with calculate_vp()

Warning: using experimental SINGLE polarization mode on S-band data, results may be unreliable!

but since this text output is rather verbose, it is easily overlooked. An informative warning from R would stand out more

[iskandari]

note: Mistnet has only been trained on S-band data, may not work with C-band. Maybe warning should be thrown in vol2bird R