Rewrote Spectrum.resample() and test_resample().

When pixel sizes are being increased a decimation filtering stage is now used before regridding, whereas the original behavior was to perform piecewise integrations for each output pixel.

When pixel sizes are being reduced, simple linear interpolation is followed by decimation filtering.

The new implementation is significantly faster, so instead of there being two unit-test functions, there is now just one unit-test function that tests both down-sampling and up-sampling. This is marked as being a fast test.