Qopen

* Qopen is now Python3-only, if you need support for Python2 use the 1.x releases
* remove parallel option, set njobs to 1 for sequential processing

* rename omM to sds (source displacement spectrum)
* site: add option to ignore some stations when aligning site responses
* allow to pin site response of a set of stations (previously only a single station or all stations)
* minor bugfixes

* core: report fit error for source displacement spectra
* core: collect_results returns dict instead of list
* imaging: labels for plots of source displacement spectra
* site: align sites now correctly aligns around the given response,
before it was shifted by a small value
* site: recalculate mean site amplifications after alignment
* site: greatly enhance test suite
* fixed some minor bugs
* other small enhancements

* move some functions from core to new source and site modules
* API documentation now available on readthedocs
* bugfix: fix plotting of source model and seismic moment in sds plots
* core: if picks are used, S picks are compulsory and P picks are optional now
* core: fix some more ObsPy depreciation warnings

* fix ObsPy depreciation warnings
* remove support for ObsPy<1.0

* bugfix: observed energy now correctly calculated with a factor of 1/2 instead of 1/4
* core: rename "correct sensitivities" to "align sites"
* core: "align sites" now only computes on largest contiguous area
* core: move free surface correction term from qopen.rt module to the calculation of observed energy
* core: free surface correction is configurable
* core: new command line option "--calc-source-params" for re-calculating source parameters
* core: use source model of Abercrombie 2015 to fit source displacement spectra and
allow to invert for M0, fc, n, and gamma
* core: add possibility to filter Inventory with Inventory.select
* core: other new, not very important, but useful command line options