Convert a soil particle phi value into diameter dia [micro-meters].
See also TT.dia2phi(). dia = (2^-phi)*1000. Not used by the package.
TT.phi2dia(phi)
Julien Moeys [aut, cre], Wei Shangguan [ctb], Rainer Petzold [ctb], Budiman Minasny [ctb], Bogdan Rosca [ctb], Nic Jelinski [ctb], Wiktor Zelazny [ctb], Rodolfo Marcondes Silva Souza [ctb], Jose Lucas Safanelli [ctb], Alexandre ten Caten [ctb]