Spectral unittest code

This is the first version of a complete spectral unit test code. The coverage of the tests is reasonable. In the next version we will see if we can make the test more deterministic. Currently we are using a random input time series and hence we check the output only to within a tolerance (the tolerance level is chosen such that the test will pass always). If there is going to be a global integration test, then some of the input/output products from those tests could be used here and that might make this more deterministic.