Test cases missing for rml:subject, rml:languageMap, rml:childMap, rml:parentMap, and the constant- and reference-valued rml:datatypeMap

[elsdvlee]

Test cases are missing for:

• rml:subject
• rml:languageMap in combination with rml:constant, rml:reference and rml:template
• rml:childMap in combination with rml:constant, rml:reference and rml:template
• rml:parentMap in combination with rml:constant, rml:reference and rml:template
• rml:datatypeMap in combination with rml:constant and rml:reference