Spectral cleaning

This merge request to address issues raised in the issue. In addition to that we have cleaned up the code a bit. Below are the main highlights of the changes, (i) More documentation added to different functions (ii) Now all the functions take inputs as 'time' intervals (previously some input to some functions were 'time' intervals while for some functions they were number of data points) (iii) Also removed the variable 'doOverlap' and now we only need overlap factor and if that factor is zero then there is no overlap (previously we had both 'doOverlap' and 'overlap_factor' which seems redundant and could lead to more confusion such setting 'doOverlap' to false and setting overlap_factor to non-zero. (iv) Now the default values for overlap_factor is 0 and window function is 'boxcar' (corresponding to no window). These changes are made in the spectral.py module. Because of the change here to keep the test_delta_sigma cut failing I have also modified it.