add module(s) of tools related to drawing diagrams of V(x), M(x), etc.

**Is your feature request related to a problem? Please describe.**
No.

**Describe the solution you'd like**
In order to visualize shear, moment, etc. it's handy to include XLscatter charts in worksheets or workbooks; however, creating and fine-tuning them includes a lot of repetitive tasks that are begging to be automated. I'll start working on a PR that adds a basic toolkit for these tasks in the form of a new module (working title = "DiagramTools") and if appropriate an associated class module once I become more familiar with the ins/outs of OOP.

**Additional context**
Some initial ideas:
* Support for multiple collinear beam elements, i.e. each member is a separate series with local-x transformed by adding .iNode.Xcoord of that member (or iNode.Ycoord for columns, or whatever is appropriate for sloped members... Pythagoras, right?).
* Automatically set min/max bounds and major/minor units of X-axis based on X coordinates of the member(s)
* Experiment with implementation of user-defined preferences for chart format characteristics such as colors, linetype, use of Excel's built-in styles, etc.