Format prismatic joint limits in DHRobot string output

[RoboX2020]

Fixes #711 (the display precision part).

str(robot) printed prismatic joint limits as raw floats, so a limit of 0.30479999999999996 (for example 12 inches converted to metres) showed up with all the float noise in both the standard and modified DH tables. Every other column already goes through format_attr.

This runs the prismatic limits through format_attr as well, so that limit now displays as 0.3048. Only the display changes. Stored limits and dynamics are untouched.

Tests: added tests/test_DHRobot_display.py, which builds a prismatic link with that limit for both PrismaticDH and PrismaticMDH and checks the output. It fails on main and passes with this change. The full test suite (excluding test_blocks.py) passes: 778 passed, 70 skipped.

This does not add the separate .table() API suggested in the issue; that would need some design discussion first.

[codacy-production]

Up to standards ✅

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🟢 Metrics 4 complexity · 0 duplication

Metric	Results
Complexity	4
Duplication	0

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