only set dendrogram display data if not previously set

Closes #485
Use qglue parsing for data loaders
Let data loaders return non-glue Data objects. Provide default file identifier
Minor typo correction
Minor typo correction
Fixing bug in docstring's code example
Merge pull request #495 from yuvallanger/patch-1

Minor typo correction
Merge pull request #496 from yuvallanger/patch-2

Minor typo correction
Merge pull request #497 from yuvallanger/patch-3

Fixing bug in docstring's code example
Merge pull request #494 from ChrisBeaumont/adass_tweaks

Convenience tweaks for data loaders
Merge pull request #493 from ChrisBeaumont/485

Quick fix for #485
Minor typo correction
Minor typo correction
Minor typo correction
Uneeded space
Minor typo and syntax correction
Minor typo correction
Minor typo correction
Changing docstring to reflect code
Merge pull request #502 from yuvallanger/patch-2

Minor typo correction