Parse LAS 3 format codes in header section lines

[kinverarity1]

Is your feature request related to a problem? Please describe.
LAS 3 extends header line items to include a format code:

I_DT .     : SURVEY_DATE                           {D}
I_CO .     : Recording Company                     {S}
I_RF .Unit : Depth Datum Elevation (from MSL)      {F}
I_AT .     : Azimuth North Type (e.g. Grid/ True)  {S}

See page 23 of the spec: https://github.com/kinverarity1/lasio/blob/main/standards/LAS_3_File_Structure.pdf

Describe the solution you'd like
They should be parsed in the relevant parts of e.g. lasio.SectionParser and lasio.read_header_line, converted where possible to Python string formatting codes, and stored and made available to the user as part of lasio.HeaderItem.