enh: new entities

* new entities: standard error, resMS, stat_image_properties
* couple of fixes for excursion_set_it.
* update of provn, json and ttl serializations and pdf and svg exports.
add: https://central.xnat.org/data/projects/PALS/subjects?format=json
Remove optional placeholders

Delete optional placeholders in relations 'used', 'wasGeneratedBy' and
'wasAssociatedWith'.
New "data scaling" entity

In agreement with NIDASH-DDWG call on December 2nd.
fix: update excursion set file name
Add separate coordinate for center of gravity
fix: still working on json-ld formatting
Merge branch 'fslUpdate' of https://github.com/cmaumet/ni-dm
Merge pull request #13 from cmaumet/newActivity_dataScaling

New entity for data scaling
Merge branch 'master' of github.com:jbpoline/ni-dm

Conflicts:
	examples/spm/spm_results.provn
	examples/spm/spm_results.svg
update svg and pdf
fix: placeholder outside parenthesis