UCM13 — Attribute values of parts cannot be set in composite component implementation, while configuration parameter values of connections can

In CompositeComponentImplementation, the ucm_base meta-model allows the specification of configuration parameter values in Connection and ConnectionEnd, but it does not allow the specification of attribute values in AssemblyPart. The meta-model would be more consistent if it supported the specification of attribute values in AssemblyPart.
For example, if we have the following component declaration:

<componentModule name="comp">
  <compType name="C1">
    <attribute mode="read" name="attr1" type="::dataTypes::Int32_t"/>
  </compType>

  <atomic name="C1_i1" lang="::ucm_lang::cpp::CPP11_typed" type="C1"/>
</componentModule>

We should be able to write something like this:

<componentModule name="comp2">
  <compType name="C2"/>

  <composite name="C2_i1" type="C2">
    <part name="c1a" ref="::comp::C1_i1">
      <attrVal ref="attr1" value="23"/>
    </part>
    <part name="c1b" ref="::comp::C1_i1">
      <attrVal ref="attr1" value="12"/>
    </part>
  </composite>
</componentModule>

Attribute attr1 shall be initialized to 23 for part c1a and to 12 for c1b.

Set attribute values in assembly parts

Add a new metaclass AttributeValue that allows the specification of a value for an attribute. This metaclass is referenced by metaclass AssemblyPart.

Also update the DTD and XML schema.