ulp_plot is slow

@NAThompson:

Test program below for erfc, but the issue is twofold:

* Condition numbers are calculated numerically - which is great when there's no choice - but terribly slow compared to calculating the derivative algebraically, especially for something like erf/erfc where the derivative is trivial: https://www.wolframalpha.com/input/?i=d%2Fdx+erfc%28x%29
* Unless I'm mistaken, we're calculating condition numbers for ever point at which we evaluate f(x), but can we not just evaluate at a few points and fit a bezier through them? Or failing that set a "stride" value, and calculate the condition value at every n'th point?

Otherwise almost all of the runtime in the program below is in thrashing through erf calls calculating condition numbers.

```

// (C) Copyright Nick Thompson 2020.
// (C) Copyright John Maddock 2020.
// Use, modification and distribution are subject to the
// Boost Software License, Version 1.0. (See accompanying file
// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
#include
#include
#include
#include
#include
#ifdef BOOST_HAS_FLOAT128
#include
#endif

using namespace boost::multiprecision;

#ifndef TEST_TYPE
#define TEST_TYPE cpp_bin_float_50
#endif

std::string test_type_name(BOOST_STRINGIZE(TEST_TYPE));
std::string test_type_filename(BOOST_STRINGIZE(TEST_TYPE));

using boost::math::tools::ulps_plot;

int main()
{
std::string::size_type n;
while ((n = test_type_filename.find_first_not_of("_qwertyuiopasdfghjklzxcvbnmQWERTYUIOPASDFGHJKLZXCVBNM1234567890")) != std::string::npos)
{
test_type_filename[n] = '_';
}

using PreciseReal = boost::multiprecision::mpfr_float_100;
using CoarseReal = TEST_TYPE;

typedef boost::math::policies::policy<
boost::math::policies::promote_float,
boost::math::policies::promote_double >
no_promote_policy;

auto ai_coarse = [](CoarseReal const& x)->CoarseReal {
return erfc(x);
};
auto ai_precise = [](PreciseReal const& x)->PreciseReal {
return erfc(x);
};

std::string filename = "erfc_errors_";
filename += test_type_filename;
filename += ".svg";
int samples = 100000;
// How many pixels wide do you want your .svg?
int width = 700;
// Near a root, we have unbounded relative error. So for functions with roots, we define an ULP clip:
PreciseReal clip = 40;
// Should we perturb the abscissas? i.e., should we compute the high precision function f at x,
// and the low precision function at the nearest representable x̂ to x?
// Or should we compute both the high precision and low precision function at a low precision representable x̂?
bool perturb_abscissas = false;
auto plot = ulps_plot(ai_precise, CoarseReal(-10), CoarseReal(30), samples, perturb_abscissas);
// Note the argument chaining:
plot.clip(clip).width(width);
plot.background_color("white").font_color("black");
// Sometimes it's useful to set a title, but in many cases it's more useful to just use a caption.
std::string title = "Erfc ULP plot at " + test_type_name + " precision";
plot.title(title);
plot.vertical_lines(6);
plot.add_fn(ai_coarse);
// You can write the plot to a stream:
//std::cout << plot;
// Or to a file:
plot.write(filename);
}
```