Compilation with cpu_CMakeLists.txt fails

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Hi,
there seems to be some missing #ifdef __USE_CPU in filtered_backprojection.cpp. The project does not build from master because several GPU specific functions aren't available.

[build] /home/andrew/workspace/LEAP/src/filtered_backprojection.cpp: In member function ‘bool filteredBackprojection::filterProjections(float*, float*, parameters*, bool)’:
[build] /home/andrew/workspace/LEAP/src/filtered_backprojection.cpp:282:37: error: ‘zeroPadForOffsetScan_GPU’ was not declared in this scope; did you mean ‘zeroPadForOffsetScan’?
[build] 282 | g = zeroPadForOffsetScan_GPU(g, params, g_out);
[build] | ^~~~~~~~~~~~~~~~~~~~~~~~
[build] | zeroPadForOffsetScan
[build] /home/andrew/workspace/LEAP/src/filtered_backprojection.cpp: In member function ‘bool filteredBackprojection::execute(float*, float*, parameters*, bool, bool, bool)’:
[build] /home/andrew/workspace/LEAP/src/filtered_backprojection.cpp:459:41: error: ‘zeroPadForOffsetScan_GPU’ was not declared in this scope; did you mean ‘zeroPadForOffsetScan’?
[build] 459 | g_pad = zeroPadForOffsetScan_GPU(g, params);
[build] | ^~~~~~~~~~~~~~~~~~~~~~~~
[build] | zeroPadForOffsetScan
[build] /home/andrew/workspace/LEAP/src/filtered_backprojection.cpp:505:33: error: ‘cudaFree’ was not declared in this scope
[build] 505 | cudaFree(g_pad);

Start with src/filtered_backprojection.cpp and the CPU configuration in cpu_CMakeLists.txt. Reproduce the CPU-only build and inspect the reported calls to GPU functions and cudaFree. Done means the project compiles successfully with the CPU CMake configuration without GPU-only symbols.

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