code refactor

related to #8 (closed):

• I kept changes per commit small, so the number of commits got kind of numerous...

tests/example_data/will_fail/output.ict

+25, 1001
+F. Obersteiner; A. Zahn; f.obersteiner@kit.edu
+Institute for Meteorology and Climate Research (IMK), Karlsruhe Institute of Technology (KIT), 76021 Karlsruhe, P.O. Box 3640, Germany
+1 | HALO | FAIRO | Ozone measured with the dual-beam UV-photometer that is part of the FAIRO instrument on board the research aircraft HALO
+HALO campaign CIRRUS-HL; DLR-IPA, Wessling, Germany
+1, 1
+2021, 07, 08, 2022, 02, 23
+0
+Start_UTC, seconds, Time_Start, number of seconds from 00:00 UTC
+2
+1.0, 1.0
+-9999999, -9999999
+Stop_UTC, seconds, Time_Stop, number of seconds from 00:00 UTC
+O3, ppb, mole_fraction_of_ozone_in_air, ozone (O3) volume mixing ratio
+8
+Flight: CIRRUS-HL_12
+Platform: HALO
+Instrument: FAIRO-1
+FileName: CIRRUS-HL_12_2021-07-08_FAIRO_O3_V1.ict
+FAIRO data processing software version: 2021.06.26a
+icartt Python package version: 1.9.1
+Exceptions To Consider: No
+ChangeLog: 0
+1
+PI_CONTACT_INFO: N/A
+PLATFORM: N/A
+LOCATION: N/A
+ASSOCIATED_DATA: N/A
+INSTRUMENT_INFO: N/A
+DATA_INFO: N/A
+UNCERTAINTY: N/A
+ULOD_FLAG: N/A
+ULOD_VALUE: N/A
+LLOD_FLAG: N/A
+LLOD_VALUE: N/A
+DM_CONTACT_INFO: N/A
+PROJECT_INFO: N/A
+STIPULATIONS_ON_USE: N/A
+OTHER_COMMENTS: N/A
+REVISION: N/A
+Start_UTC, Stop_UTC, O3
+12.300000, 12.500000, 2378942320000.000000
+12.600000, 13.100000, 32423564410000.000000
+13.400000, 14.000000, 234892500000.000000
+14.100000, 14.900000, 2342563400000.000000
+15.400000, 15.000000, 5245249529000000.000000