Metal 3.1.0

The project has been changed into a Maven multiple module project. Which instead of just metal now has two artifacts: metal-core and metal-formats. The former replaces the original artifact and the latter contains format descriptions that can be used by client applications or as examples. To continue using the core library, update the dependency to:

<dependency>
  <groupId>io.parsingdata</groupId>
  <artifactId>metal-core</artifactId>
  <version>3.1.0</version>
</dependency>

This release also brings three additions to the core API (1-3) and a small internal change (4):

1. Added ByToken.getAll() to retrieve all ParseItems resulting from a provided Token (#22 by @ccreeten)
2. Added Elvis operator as ValueExpression as an alternative to a standard ternary (#34 by @rdvdijk)
3. Added Len ValueExpression to return the length in bytes of a provided ValueExpression (#37 by @gertjanal)
4. Removed some unnecessary dependencies on the Java standard library (#17 by @rdvdijk and @jvdb)