Schema: allow Case in ExampleLike

[jjrsylvestre]

This one is a little more controversial.

Current schema is

ExampleLike =
    MetaDataTitleOptional,
    (
        (BlockStatement)+ |
        (Statement, Hint*, Answer*, Solution*) |
        (IntroductionStatement?, Task+, ConclusionStatement?)
    )

but would be nice if that first (BlockStatement)+ were changed to (BlockStatement | Case)+ to allow example calculations to be broken up into cases.

Example use case:

Example 1. In this example we will determine the domain of continuity of the piecewise function f(x) = ..... We will do this by searching for points of discontinuity on the various subdomains of the function; discarding those discontinuities leaves the domain of continuity.

Case x < a. [Details.]

Case of transition point x = a. [Details.]

Case a < x < b. [Details.]

Case of transition point x = b. [Details.]

Case x > b. [Details.]

Putting this all together, the domain of continuity of f(x) is ....