Add improvesQualityIndicator for SingularityCE

[srghosh56]

Resolves #200

Adds improvesQualityIndicator for SingularityCE

Reasoning for each indicator chosen:

• requirements_specified — A Singularity .def file explicitly declares all system requirements and dependencies
• dependency_management — Containers encapsulate and freeze all dependencies that helps SingularityCE to enable dependency management
• software_has_documentation — Definition files and container recipes serve as reproducible, self-describing documentation of the software environment
• versioning_standards_use — SingularityCE images are tagged and versioned promoting adoption of versioning standards
• has_releases — Building and publishing SIF image files is fundamentally a release activity
• descriptive_metadata — Container images and their definition files carry descriptive metadata about the software environment
• has_published_package — SingularityCE produces a downloadable SIF (Singularity Image Format) file which is a published, distributable package