Image maf files

[proccaserra]

a PR to resolve issues #605 (metabolight assignment file) and #375 (Image File)

test data made available via the dedicated repository (ISAdatasets) on tests branch, covering both ISA-tab and ISA json serialisations.

Note:

• Supporting the 'Image File' only works if a new ISA Configuration File is added to the default ISA configuration. The configuration file defines a new assay where 'measurement type' = 'phenotyping analysis' and 'technology type' = 'imaging'.
• More advanced ISA Configuration may be added to cover various microscopy imaging procedures such as the cell migration assay using microscopy imaging:
  https://github.com/CellMigStandOrg/MIACME/blob/master/v1.1/ISATab-configurations/isaconfig-MIACMEv1.1/cellmigration-imaging-MIACMEv1.1.xml

[coveralls]

coverage: 82.526% (+0.05%) from 82.478%
when pulling 25a20c0 on image-maf-files
into 5ad584c on develop.