Added unit tests for the Spectrum and SpectralVariance
This PR introduces a new TestSuite for the Spectrum object, then fixes a few bugs in that object uncovered by the unit tests (fixes #236 (closed)), and does the same for SpectralVariance.
This PR introduces a new TestSuite for the Spectrum object, then fixes a few bugs in that object uncovered by the unit tests (fixes #236 (closed)), and does the same for SpectralVariance.