Fix partial load span bug

[cding91]

@SaMohamed FEALiTE2D is a wonderful library for frame analysis. I have used it in two of my engineering software projects. I particularly like FEALiTE2D's internal mesh segment. It makes meshing so simple. It automatically considers loading discontinuity using mesh segments. I think this a big differentiating factor.

While using LinearMesher to handle internal meshing, I encountered bug. I have taken some time investigating it and provided a fix for it.

Bug

Null pointer exception is raised under the following scenario. When I apply uniform load over a frame element only on part of the span, I call GetElementInternalForcesAt with position set beyond where the uniform load ends. For example, see below.

I have added some tests for PostProcessor class . The test GetElementInternalForcesAt_WithPartialUniformLoad_EdgeSegment_ReturnsCorrectForce will trigger the bug.

Cause

I think the root cause is GetElementInternalForces when the library calculates end responses of each mesh segment. To calculate the responses, I believe the library just uses all the loads on the left of the mesh segment left end. For frame unfiorm load, FrameUniformLoad uniformLoad = uL.GetLoadValueAt(element, x) as FrameUniformLoad; may result into null reference.

Fix

I believe uL.GetLoadValueAt(element, x) as FrameUniformLoad is really just for coordinate transformation. Passing uL.L1 instead of x can serve the purpose and avoid null reference problem.

Docs

I suggested adding a github page documentation for this library. I have taken liberty to create one just my company to use. It is auto generated by DocFx. I am happy to transfer the page to this repo.