Improve PiecewiseExpand

[axkr]

Improve PiecewiseExpand function for example for nested Piecewise functions...

Implement more cases in this method:

• symja_android_library/symja_android_library/matheclipse-core/src/main/java/org/matheclipse/core/builtin/Arithmetic.java

  Line 6147 in a08cee3

  public static IAST piecewiseExpand(final IAST function, IBuiltInSymbol domain) {

• create new JUnit test cases to test the implementation.

• create markdown documentation for 'PiecewiseExpand`

See:

• https://github.com/axkr/symja_android_library/blob/master/symja_android_library/doc/functions/Piecewise.md

Discuss here:

• https://gitter.im/symja_android_library/Lobby