plot_piecewise: show jumps of sign, Heaviside, floor, ceiling and frac, and holes

[inayatku]

plot_piecewise draws filled and empty dots at the ends of the pieces of a Piecewise expression. Functions with jumps that are not written as Piecewise were drawn with vertical lines and no dots. Removable discontinuities were not shown.

This PR:

• rewrites sign, Heaviside, floor, ceiling and frac as Piecewise over the plotting range before the pieces are processed. floor, ceiling and frac are rewritten only when their argument is linear in the plotting variable and the range covers at most 100 integer steps;
• removes from the domain the points where the expression is undefined but both one-sided limits are finite (zeros of a polynomial denominator). These points get empty dots: the hole of (x**2 - 1)/(x - 1) at x = 1, sin(x)/x at x = 0, and the two ends of Abs(x)/x at x = 0. Poles are not changed;
• fixes the offset at the open ends of pieces shorter than one unit (separate commit). offset *= e gave a negative offset, so the piece was evaluated outside its interval, and the empty dot could be NaN. For example, Piecewise((sqrt(1/2 - x), x < 1/2), (1, True)) lost its empty dot at x = 1/2.

Before and after, with plot_piecewise(expr, (x, a, b)):

Before:

After:

Tests: four new tests in tests/plot_functions/test_functions_2d.py. With the change, tests/graphics, tests/plot_functions, tests/test_series.py, tests/test_utils.py and tests/backends/test_matplotlib.py give 562 passed, 206 skipped and 4 xfailed (558 passed before, the same skips and xfails). The existing doctest examples of plot_piecewise give the same output.

Related SymPy issues: sympy/sympy#17738 and sympy/sympy#26328.

[inayatku]

Hi @Davide-sd, a gentle reminder about this pull request. The CI workflows are waiting for approval, since this is my first contribution here. The first commit (the offset *= 10**e fix in _process_piecewise) is independent of the rest. I can move it to a separate pull request if that makes the review easier. Thank you for your time.