Change model and doc from name SimpleAllele to AlleleInstance

[larrybabb]

All changes have been made (looking for volunteer to verify).

• resource/conceptual page content and links
• folder names
• examples
• model (graphics)
• references from assertion docs
• discussion pages
• scripting references

All references to the terms "simple allele", "SimpleAllele" and many variations thereof have been eradicated from the documentation and repo in order to remove the confusion being caused with the SimpleCanonicalAllele term.

[larrybabb]

@srynobio will check through the site and verify that all references have been removed/changed appropriately.