Add filter combinator

Fixed to take partial function as a predicate
Simple filter goes sequentially from the start whenever needed
Modify showcase examples

Pedagogical examples corrected to test dates enumerator (from the paper)
Binary search tree showcase confirmed for compilation
Other sources refactored
Modify SciFe DSL

Added/fixed implicit conversions
 (Note that streamToEnum now returns Infinite to resolve implicit conversions
 where stream was converted to Singleton)
Implicit conversions ordering
Binary Search Tree showcase example now compiling and tested
Merge branch 'feat/showcase' into develop