Error due to change in default stringsAsFactors settings

[davidcsterratt]

A number of checks are broken in R-dev due to a change in the behaviour of stringsAsFactors (see https://developer.r-project.org/Blog/public/2020/02/16/stringsasfactors/index.html).

  > library(testthat)
  >
  > test_check("retistruct")
  Loading required package: retistruct
  -- 1. Error: IDT format is read correctly (@test-format-idt.R#9) --------------
  non-numeric argument to binary operator
  Backtrace:
   1. retistruct::retistruct.read.markup(r)

  -- 2. Error: Reconstruct SMI32 (CSV format) (@test-reconstruct.R#8) -----------
  non-numeric argument to binary operator
  Backtrace:
   1. retistruct::retistruct.read.markup(o)

  -- 3. Error: Reconstruct GMB530/R-CONTRA (IDT format) (@test-reconstruct.R#45)
  non-numeric argument to binary operator
  Backtrace:
   1. retistruct::retistruct.read.markup(o)

  -- 4. Error: Serialisation works with a particular example (@test-reconstruct.R#
  non-numeric argument to binary operator
  Backtrace:
   1. retistruct::retistruct.read.markup(a, error = message)

  -- 5. Error: No regression on Issue #21 (@test-regressions.R#5) ---------------
  non-numeric argument to binary operator
  Backtrace:
   1. retistruct::retistruct.read.markup(a)