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.
Merge branch 'master' of github.com:jbpoline/ni-dm

Conflicts:
	examples/spm/spm_results.provn
	examples/spm/spm_results.svg