Added new actions to vdb tool

[kmuseth]

New actions in vdb_tool:

Stack management
-copy — deep-copies VDB grids and/or Geometry by index onto the top of their stacks. Supports comma-separated indices, * wildcard, and an optional prefix= to rename copies. Defaults to vdb=0 (top grid).
-rename — renames a VDB grid and/or Geometry on the stack by age index.
-swap — swaps two VDB grids and/or two Geometry entries. O(1), no data copied.

Composite math (via tools::compMul / tools::compDiv)
-multiply (alias -mul) — per-voxel product of two scalar grids.
-divide — per-voxel quotient of two scalar grids.

Inspection
-stats — prints a value-derived table per grid: background, min, max, mean, std. dev., and for level sets: world-space surface area and enclosed volume. Intentionally complementary to -print which is structural-only.
-diagnose — runs tools::checkLevelSet / tools::checkFogVolume / NaN+Inf checks per grid. Prints OK or FAILED with indented messages. Options: vdb=, checks=N (default 9), fatal=true.

Changes to existing actions
-print — removed level=2 (background, value range, area, volume moved to -stats). Now level=0 and level=1 only.
-copy — vdb defaults to 0 so -copy alone duplicates the top grid.
-diagnose — gained checks=N option to control how many checks to run.

Bug fixes
Diagnostics.h — fixed a floating-point false positive in CheckLevelSet::checkBackground() where background / voxelSize could compute to just below LEVEL_SET_HALF_WIDTH due to IEEE 754 precision, causing a correctly-created level set to fail check #4. Fixed with a relative tolerance of 1e-6. Regression test added to TestDiagnostics.cc.

Config file improvement
Backslash line continuation in config files — a line ending with \ (trailing whitespace is ignored) is joined to the next line before tokenization, allowing long action invocations to be split across multiple lines. Unit test added (ToolConfigLineContinuation).

CMake fixes
vdb_tool/CMakeLists.txt — fixed OpenEXR v2/v3 compatibility: checks for OpenEXR::OpenEXR (v3) first, falls back to OpenEXR::IlmImf (v2). Also added find_package(Imath) + Imath::ImathConfig to propagate Imath include paths, fixing a build failure on systems where OpenEXR 3.x's headers use bare <ImathBox.h> includes.

Documentation
README.md — added actions to the table (copy, diagnose, divide, multiply, rename, stats, swap); updated -print and -stats descriptions to reflect the structural vs value-derived distinction; added a new "Inspecting grids: -print vs -stats" section; documented backslash line continuation in the config file format section.
pendingchanges/openvdb_cmd.txt — added entries for all new actions, updated -print and -stats descriptions, added the Diagnostics.h float fix entry.