ENH: adjustments to Switch

- align Switch more with Enum.
  Now have find(), found() static methods.
  Constructors with failsafe option.

  The find() method makes for clearer coding:

  OLD

     Switch sw(some_string, true); // NB: true = allowBad

     if (sw.valid()) ...

  NOW

     Switch sw = Switch::find(some_string);

     if (sw.good()) ...

  or

     if (Switch::found(some_string)) ...

- improve construct from dictionary to handle all valid token types.
  Previously just read in a word.

- Remove asText() method - replaced by c_str() and str() several
  versions ago.