Fix PMF maximum window size precision

[tianrking]

Summary

• store ProgressiveMorphologicalFilter's maximum window size as float
• document that PMF uses input-coordinate units while ApproximateProgressiveMorphologicalFilter intentionally uses grid-cell counts
• add a regression test that checks both the API type and preservation of a fractional value

Motivation

PMF computes each window size as cell_size * (2 * base^iteration + 1) and passes that coordinate-space value to the morphological operator. Comparing it with an integer maximum truncates valid fractional thresholds. The approximate implementation has different, existing semantics: its maximum is a number of cells, so this change deliberately leaves that API as int.

Fixes #6414

Testing

• built and linked the new test_progressive_morphological_filter target with GCC 15.2 and PCL_ONLY_CORE_POINT_TYPES=ON
• confirmed a baseline type probe fails because getMaxWindowSize() returns int, while the updated API compiles as float
• ran a PCL_NO_PRECOMPILE runtime probe; setting 6.25f returned 6.25
• g++ -std=c++17 -fsyntax-only test/segmentation/test_progressive_morphological_filter.cpp with the configured PCL include paths
• clang-format --dry-run --Werror test/segmentation/test_progressive_morphological_filter.cpp
• git diff --check