> install_conversion_offset("tempK", "my_temp_C", 273.15)
ut_offset(): NULL unit argumentError in R_ut_offset(as.character(to), as.character(from), as.double(const)) : 
  Error in function R_ut_offset: An argument violates the function's contract

make errors friendlier, so it becomes clear what is wrong.
1. first arg does not exist
2. second does already exist


help of ?set_units: ... ; mention NSE


# This should not occur:
> units(b) = "10 m"
> b
Units: ^10*m
[1] 1 2 3


DONE
=======================
search path for udunits2.xml:

/usr/local/share/udunits/udunits2.xml
/usr/share/udunits/udunits2.xml 
/usr/share/xml/udunits/udunits2.xml
/usr/share/udunits/udunits2.xml


fallback:
  system.file("share/udunits2.xml", package="units")

set_units(1, g/kg)
-> errors
-> retain g/g ratio as special case of unitless?


# This should be an error:
> b = 1:3
> units(b) = "10*m"
Warning message:
In ‘10 * m’ the numeric multiplier ‘10’ was discarded. 
The returned unit object was coerced to a value of 1.
Use `install_conversion_constant()` to define a new unit that is a multiple of another unit. 
=== resolved

# This should not occur:
> set_units(1:3, as_units("m"))
Units: 1
[1] 1 2 3
==== resolved
