MAGEtab export: better Unit support

[agbeltran]

Reported by @sneumann as ISA-tools/ISAcreator#283

I have a factor "Sampling time", which is defined in the Investigation,
and added to the s_Study.txt with Units "hour" from UO:

"Factor Value[Sampling time]"   "Unit"  "Term Source REF"       "Term Accession Number"
"6"                             "hour"  "UO"                    "http://purl.obolibrary.org/obo/UO_0000032"

This gets exported into sdrf.txt as:
"Factor Value[Sampling time]" "Unit[UnknownUnitType]"

while it should be: Unit[TimeUnit] for ArrayExpress.

Yours,
Steffen