Implemented final optional vs obligatory attributes in SimpleTag.

SimpleTag now requires "position" as truly obligatory field, expecting it to be set upon construction. Therefore "createSimpleTag" & "getSimpleTag" in "Block" and the constructors have been changed.
All tests have been updated to work with new versions.
Minor fixes to prior branch (affecting Dimensions) included.
Implemented final optional vs obligatory attributes in DataTag.

DataTag now requires "positions" as truly obligatory field, expecting it to be set upon construction. Therefore "createDataTag" & "getDataTag" in "Block" and the constructors have been modified.
All tests have been updated to work with new versions.
Minor fixes to prior branch (affecting SimpleTag) included.
Major bug fix in "EntityWithSources" that prevented "Source"-getter from working!
Implemented final optional vs obligatory attributes in Representation.

Representation now requires "data" & "linkType" as truly obligatory fields, expecting them to be set upon construction. Therefore "createRepresentation" & "getRepresentation" in "SimpleTag" & "DataTag" and the constructors have been updated.
All tests have been updated to work with new versions.
Implemented obligatory "type" attribute in NamedEntity ctor.

NamedEntity now requires "type" as truly obligatory field, expecting it to be set upon construction. Therefore numerous instances of its' use had to be modified.
All tests have been updated to work with new versions.
Removed all tabs.
Final minor fixes to getter & setter, new "MissingAttr" exception.

In particular fixed bug where obligatory getter would not throw error when attribute or entity was missing.
Introduced new exception "MissingAttr" to be thrown when an obligatory getter does not find required attribute.
Fixed some setter not throwing messages, in particular fixed "link" setter that would remove attribute falsly.
Homogenized existence of "hasXYZ" methods, removed where necessary.

"hasXYZ" methods now exist for every attribute except for optional attributes with the simple pattern "getter:attribute(), setter:attribute(...), deleter:attribute(none_t t)". For those "hasXYZ" has been removed where existing, since "attribute()" can be used instead.