Ordered contexts

[jana-fischer]

Provide the additional option to work with contexts with ordered objects and attributes.
e.g., keeping the order when reading a context from a file

Thoughts on the task:

• The Ordered-Context object consists of a set of objects, a set of attributes, the incidence relation (either as set or as function) and the orders on objects and attributes. The orders on objects / attributes are given separately to allow using existing functions from 'fca/contexts.clj'. (In detail, these are all functions in 'fca/contexts.clj' that do not call one of the constructors make-context-nc and make-context.)
• There are some functions in 'fca/contexts.clj' that use make-context-nc or make-context, e.g., rename-objects. An idea is to wrap these functions such that:
  ordered-context is converted to formal-context -> function is used on formal-context -> result is converted back to ordered-context (changes on order of objects and attributes might be required).
• Functions for read and write (I/O) needed.
• What about many-valued-contexts?

(Current status: branch 'ordered-contexts' linked under 'Development'. Constructors, some functions and tests are already written.)