Handle edge cases in linspace() for num < 2

[leofernandezg]

Summary

• Fixes ZeroDivisionError when calling linspace() with num=1

Problem

linspace(start, stop, num=1) computes step = (stop - start) / (num - 1), which divides by zero. This function is used internally by roundcorners() and SegmentArc.get_bbox().

Changes

• schemdraw/util.py: Return [start] when num=1 and [] when num=0, consistent with numpy.linspace behavior
• test/test_linspace.py: New test file covering num=0, num=1, num=2, and default cases

Test results

Without fix: test_linspace_num_1 fails with ZeroDivisionError
With fix: all 4 tests pass

Fixes #93

[leofernandezg]

Test suite results

Full test suite (py.test --nbval-lax test/) run on this branch produces the same results as master:

Branch	Failed	Passed
master (baseline)	38	432
this branch	38	436

The 38 pre-existing failures are in test_parselogic.ipynb and test_timing.ipynb (unrelated to this change).

[leofernandezg]

Update: Re-ran the full test suite with all optional dependencies installed (matplotlib, ziamath, pyparsing).

Branch	Passed	Failed
master	470	0
fix/linspace-division-by-zero	474	0

No regressions introduced.