BARRA-R: conversions required to comply with ALMA/CF

[aukkola]

• Variable names to be changed to ALMA convention
• Units to be checked
• Some variables are reported at model levels, need to pick the lowest model level: cld/rain, mdl/air_temp, mdl/pressure, mdl/spec_hum and mdl/wnd_ucmp and mdl/wnd_vcmp
• wind speed provided as u- and v-components, need to calculate average wind speed from these
• multiple .nc files provided for each day, need to combine into annual files
• time units are "hours since 1970-01-01 00:00:00", might need tweaking

Data for BARRA v1 is located on project cj37 on NCI. BARRA v2 is also available on ob53 (it might still be a pre-release but might be worth investigating if we should use it instead)