unable to set multi-value context without warnings

[lsloan]

When emitting an event with custom entities (e.g., a custom action), it's usually necessary to set a new context, or an additional one, on the event to define that entity. There are a couple of problems with that, however.

1. events\Event::setContext only allows a single value to be set. It doesn't allow multiple values. It should allow multiples.

2. As a workaround to the problem above, a custom context can be defined with an array of context URIs as its value.

   class CustomContext extends Context {
   	const
   		__default = [
   		'http://example.edu/caliper/context/v1p1_custom.json',
   		'http://purl.imsglobal.org/ctx/caliper/v1p1'];
   }
   
   $event = (new Event())
   	->setContext(new CustomContext());

   However, when that event is serialized to JSON, warnings are raised: Warning: Array to string conversion in umich-caliper-php/src/util/BasicEnum.php on line 104. That occurs when entities' contexts are compared to those of the event. util\BasicEnum::__toString and probably the code making the comparisons need to be updated to support multi-value contexts.

[lsloan]

Maybe context doesn't need to be set using enum. Maybe they should just be simple strings.