flex_extract templates with reduced level number

(trac ticket #315 by Petra) For most applications, it is not necessary to extral al of ECMWF's IFS model levels. Usually levels covering troposphere and lower stratosphere are sufficient.

Benefits:

• less data to transfer and store
• FLEXPART runs will require less memory
• FLEXPART runs will require less CPU time

Therefore, suitable standard templates should be distributed with flex_extract