Add checks to docs + data validator that R >= 0

[mgovers]

We almost no-where in our docs specify that r >= 0. We do specify that r and x cannot be both 0.0.
This may lead to multiple places in our code having UB in which we do not check for negative values of r.

The only places where we do check that r >= 0 is:

• for source (via rx_ratio and z01_ratio)
• for asymmetric line
  • TBD: we also require that x > 0 and c > 0. Is that correct?
• for automatic tap regulator (line drop compensation)

Originally posted by @mgovers in #1197 (comment)