Introduced specialized AttributeDictionary

Ideally, models will optionally accept a dictionary of attributes so that scalar properties can be quickly and easily assigned without relying on reflection, in order to improve performance. The `AttributeDictionary` class will not only provide a friendlier signature than e.g. `IDictionary<string, string>`, but will also provide a friendlier `GetValue()` method for getting a value if there's a match, and otherwise returning `null`.

This will aid in supporting Feature #99.

Author: JeremyCaney

OnTopic/Attributes/AttributeDictionary.cs

@@ -0,0 +1,48 @@
+/*==============================================================================================================================
+| Author        Ignia, LLC
+| Client        Ignia, LLC
+| Project       Topics Library
+\=============================================================================================================================*/
+using OnTopic.Mapping;
+
+namespace OnTopic.Attributes {
+
+  /*============================================================================================================================
+  | CLASS: ATTRIBUTE DICTIONARY
+  \---------------------------------------------------------------------------------------------------------------------------*/
+  /// <summary>
+  ///   Provides a light-weight dictionary of attribute values.
+  /// </summary>
+  /// <remarks>
+  ///   The <see cref="AttributeDictionary"/> is used by the <see cref="TopicMappingService"/> to support self-constructed
+  ///   models that don't require the use of the <see cref="TopicMappingService"/> to set the values of scalar properties. Any
+  ///   model that has a primary constructor accepting a single <see cref="AttributeDictionary"/> will be initialized using a
+  ///   <see cref="AttributeDictionary"/> containing all attributes from not only the current <see cref="Topic"/>, but also any
+  ///   <see cref="Topic.BaseTopic"/>s it references.
+  /// </remarks>
+  public class AttributeDictionary: Dictionary<string, string?> {
+
+    /*==========================================================================================================================
+    | CONSTRUCTOR
+    \-------------------------------------------------------------------------------------------------------------------------*/
+    /// <inheritdoc/>
+    public AttributeDictionary(): base(StringComparer.OrdinalIgnoreCase) { }
+
+    /*==========================================================================================================================
+    | METHOD: GET VALUE
+    \-------------------------------------------------------------------------------------------------------------------------*/
+    /// <summary>
+    ///   Gets an attribute value as a string from the <see cref="AttributeDictionary"/> based on the <paramref name="
+    ///   attributeKey"/>.
+    /// </summary>
+    /// <param name="attributeKey">The key of the attribute to retrieve.</param>
+    /// <returns></returns>
+    public string? GetValue(string attributeKey) {
+      Contract.Requires<ArgumentNullException>(!String.IsNullOrWhiteSpace(attributeKey), nameof(attributeKey));
+      TryGetValue(attributeKey, out var value);
+      return String.IsNullOrWhiteSpace(value)? null : value;
+    }
+
+
+  } //Class
+} //Namespace