Add a system.file() example that the user could use

[russHyde]

At present, the example code in the dupree README couldn't be reran by a user if they downloaded dupree from CRAN, say, since they would require the dupree source code for the examples to work (and dupree isn't particularly bad for duplication within the source code).

It would be useful to have a couple of dupree examples that use example files (from inst/) that CRAN users could access

• - add two example files to inst.
• - add example sections that use these system.files to the dupree() roxygen
• - use the system.files within a vignette or in the README.
• - also (but unrelated) show how to visualise a similarity network using tidygraph in the vignette

[russHyde]

Have added a single example file that is used in the dupree::dupree() examples.

Further extension of the docs would be valuable