TriangularMatrix uses wrong base class

[bburlacu]

Since the underlying storage is an array, the TriangularMatrix<T> class should inherit from ValueTypeArray<T> instead of ValueTypeMatrix<T>. This would lead to a more efficient implementation.

Optional enhancements:

• option to specify storage order (row- or column major)
• a property to access the underlying T[] array

[bburlacu]

r17831: Use ValueTypeArray as base class for TriangularMatrix.

• Data property exposes the underlying array as a ReadOnlyCollection.

• No need to specify storage order since the matrix is symmetric.

[gkronber]

The TriangularMatrix is not used yet in HeuristicLab core source code. Therefore, the changes should be fine. However, one must be very careful with persistence when changing the class hierarchy (see ​https://github.com/heal-research/HEAL.Attic/blob/master/docs/Manual.md#backwards-compatibility-examples). Additionally, a storable field has been changed. Persistence (compatibility of old files) should be tested carefully.

[gkronber]

Reviewed r17831. Can be merged to stable.

[gkronber]

Change has been made to the branch for #3087. I'm closing this ticket as r17831 should be merged together with #3087.