The GreekOrthography abstraction

GreekOrthography is an abstract type, which in turn is a subtype of the Orthography package's OrthographicSystem.

Implementations of GreekOrthography can therefore:

1. use the functions specific to implementations of GreekOrthography to remove and add accents to words, to break words into syllables, and to sort words according to the logic of the Greek alphabet
2. use the generic functions of the Orthography package to assess the validity of characters and strings, and to tokenize a string into a series of explicitly classified tokens.