mosaic() should handle area units

[joeroe]

Currently, mosaic() relies on the user to specify an area parameter that matches the measurement units used in of frame and silently discards the unit class returned by sf::st_area().

It would be better if it allowed a units object to be passed to area and intelligently handled unit conversions. Assuming the units of area if it is given as a plain number should be a fallback option accompanied by a warning.