Add mappings to WMO

[cmungall]

Example WMO IRI: http://codes.wmo.int/bufr4/codeflag/0-20-012/9

from cloud types: http://codes.wmo.int/bufr4/codeflag/0-20-012

pred	val
FXY	020012
label	Cumulonimbus (Cb)
notation	9
type	Concept

Note that we already have some mappings from envo cloud classes to codes/symbols like "Cb" - see #1417

envo query -q "SELECT subject,value FROM statements where predicate='oio:hasAlternativeId' and value NOT LIKE '%:%'"

subject	subject_label	value
ENVO:01001160	cirrus cloud	Ci
ENVO:01001407	cirrocumulus cloud	Cc
ENVO:01001408	cirrostratus cloud	Cs
ENVO:01001409	altocumulus cloud	Ac
ENVO:01001410	altostratus cloud	As
ENVO:01001411	nimbostratus cloud	Ns
ENVO:01001412	stratocumulus cloud	Sc
ENVO:01001413	stratus cloud	St
ENVO:01001414	cumulus cloud	Cu
ENVO:01001419	cumulonimbus cloud	Cb

I don't actually see "Cb" as a first-class code on the WMO site.

Proposal:

Replace the altIds above with hasDbXrefs to http://codes.wmo.int/bufr4/codeflag/0-20-012/NNN (note that in future xrefs may be migrated to skos)

Variant proposal: if we want to keep the two letter codes we should find some other property to model them, but we need to find their referents in the WMO system. The URL provided in #477 no longer resolves

Extension: map other entities other than clouds