Enable XML leaf elements

[JR-1991]

This PR introduces the ability to specify leaf elements for XML data models. A leaf element is a terminal string that is encapsulated by the given element. This means, that there are no other elements in the element other than XML attributes. For instance, the following example demonstrates a use case:

XML output

<leafElement someAttr="XYZ">
    This is a simple leaf element
</leafElement>

Within the markdown specs, a leaf element can be configured by setting XML: <Name of the class>, which will signal the sdRDM compiler that the given attribute is to be used as a leaf element. Please note, the XML alias has to have the same name as teh class. Otherwise, a new element will be created.

Markdown

### LeafElement

- value
   - Type: string
   - XML: LeafElement
- someAttr
   - Type: string
   - XML: @someAttr

(Generated code will be the same as usual)

TLDR

• Allow leaf XML elements
• Specified by XML: <Class name> --> Turns attribute to function as desired
• Added tests for leaf element

Closes

closes #20