Variable name aliases

Climix should be able to handle well-known alternatives (aliases) for the standard CMIP variable names:

CMIP	alternatives (case insensitive)
tas	tasadjust, t2m, t2mean, tmean, meant, tm, tg
tasmax	tasmaxadjust, t2max, tmax, maxt, tx
tasmin	tasminadjust, t2min, tmin, mint, tn
pr	pradjust, prec, precip, rr	conversion of standard name and unit might be necessary

This initial list should be easy to extend, both with respect to new variables and new alternatives.