PartialNonuniformLoad support TriangleElement

[epsi1on]

add support for PartialNonuniformLoad into TriangleElement.
Add example to the documentation:

https://github.com/BriefFiniteElementNet/BFE.Net.devdocs/blob/master/docs/loads/elementLoads/partialnonuniformload.rst#id1