document how wis2box-user can ensure originating centre and subcentre is encoded into produced BUFR

[maaikelimper]

During the recent WIS2 Training in Brazil, the question of how to correctly encode the BUFR_ORIGINATING_CENTRE and BUFR_ORIGINATING_SUBCENTRE into the produced BUFR was again brought up.

We had some discussion about this previously in this issue:
#536

I think the issue was closed after deciding to set these to MISSING by default but there is no documentation on this in wis2box

@david-i-berry can you comment on this ?

[david-i-berry]

This was fixed and documented here: https://csv2bufr.readthedocs.io/en/latest/installation.html#environment-variables. We need to add more info and also bring it over to the WIS2box docs as well.

[maaikelimper]

OK thanks, so this is a documentation issue, I will update the title and label accordingly

[maaikelimper]

Setting environment variables in the wis2box.env will apply to all datasets shared by the wis2box-instance. Is it also possible to define these per dataset ? (this will be required by Caribbean wis2box but possibly also for others)