Add 3-hour frequency data

[larsbuntemeyer]

closes #110

Add new 3-hourly data entries (zg*, ta*, hus*, ua*, va*, wa*)

[larsbuntemeyer]

@sol1105 Sorry, i missed the "etc.". You wanted all single pressure variables from the 6hr table also in the 3hr table, right?

[sol1105]

@larsbuntemeyer Thanks for adding the variables 😃
Yes, we would require all single pressure variables from the 6hr table to be added to the 3hr table:
zg*, ta*, hus*, ua*, va*, wa*