Temperatures are expressed in Kelvin and therefore cannot be negative in
absolute value. However, you may wish to perform calculations relative
to a given temperature T. In this case, the temperatures managed by
Stardis would be relative to T and could therefore be negative, since
they would express a deviation from T. Hence this commit, which
authorizes negative temperatures.

This is a major break in the API that callers MUST take into account.
Until now, negative temperatures were considered as unknown
temperatures, whereas they are now valid. For example, an interface with
a negative temperature could be considered adiabatic, whereas it is now
a Dirichlet boundary condition. In other words, the same data could
define totally different systems before or after this modification.

A new Stardis macro is added to define the unknown temperature value.
Two help macros are also provided to test whether the temperature is
known or not.

All tests have been updated accordingly to handle this API break. It
should be noted that while the complete code has been compiled without
error, the tests have still not been re-run since this update.