Establish membershipObject() method for Interval classes

[wdcraft01]

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The number/sets/real context has recently been updated, but the IntervalOO, IntervalOC, IntervalCO, and IntervalCC classes can continue to be updated and elaborated, in particular with membershipObject() methods and a related IntervalMembership class analogous to the membershipObject() methods and XMembership classes established for the Intersection, Union, and Exp classes (for example). Such elaboration of the IntervalXX classes will help augment the automation of proofs of claims such as 2 ∈ [1, 3). Some initial discussion would be helpful to determine a collection of appropriate methods to include in the IntervalMembership class (probably housed in a file such as interval_membership.py in the number/sets/real context). Related Issue: #116.