Mirroring of a mapping set should honour the `MIR` variable

[gouttegd]

Declaring a SSSOM set in maintenance mode as follows:

sssom_mappingset_group:
  products:
    - id: fbbt
      maintenance: mirror
      mirror_from: http://purl.obolibrary.org/obo/fbbt/fbbt-mappings.sssom.tsv

yields the following code in the generated Makefile:

$(MAPPINGDIR)/fbbt.sssom.tsv:
    wget "http://purl.obolibrary.org/obo/fbbt/fbbt-mappings.sssom.tsv" -O $@

in which the mapping set is downloaded unconditionally.

Mirroring a mapping set should obey to the same rules as mirroring an imported ontology. In particular, when MIR is set to false, no attempt to download the remote set should be made.

[matentzn]

Agreed!