SHA1 or SHA256 for file checksums and Provenance document

Hello everyone,

This is continuation of the discussion initiated by [Peter with respect to CWL viewer](https://github.com/common-workflow-language/cwlviewer/issues/173). We (@stain) are currently working on generating a PROV document for every run of a CWL workflow when executed by cwltool (cwl-runner).

The checksums generated for the output files by cwltool are using SHA-1 (according to CWL v.1 specs this is the only supported version) whereas we are planning to use SHA-256 in the PROV document to refer to some resources. @mr-c and @tetron Is it possible we can generate SHA-256 checksums for the output files as well in order to be uniform? Or for the sake of simplicity of reference implementation we are planning to keep it this way? If it is not possible, we will look for other ways (may be conversion of the checksum or using SHA1 in the PROV document). All suggestions are welcomed and appreciated.

Thanks