Describe the bug

The Approx type has an overload of template <typename T, ...SFINAE...> Approx operator()(T const&) which (correct me if I'm wrong) is meant to be a factory function for instances that have the same epsilon, margin, and scale, but that use the passed value.

AFAICT this should be const on the instance, but it's not.

Minimum failing example:

#include <catch2/catch.hpp>

TEST_CASE("Approx factory is const-correct") {
  // Set up a template Approx with problem-specific margin, etc.
  Approx const apprx = Approx(0.0).margin(1e-6);
  double value = 1.0;
  // Use template in assertions
  REQUIRE(value == apprx(1.0));
}

Expected behavior
Above test compiles, runs and passes.

Reproduction steps
See above.

Platform information:

• OS: RHEL 8
• Compiler+version: GCC 8.2.0
• Catch version: 2.13.6

Additional context
Add any other context about the problem here.