Apsg

* paleomag rs3 input/output improved
* Simple SQLAlchemy API to sdb database implemented
* StereoNet arc method fixed
* StereoNet polygon method added

* Python 2 support dropped
* RosePlot added
* Ortensor has from_pairs method for Lisle tensor for orthogonal data
* StereoNet scatter method has labels kwarg to show hover annotations

* Python 2/3 compatibility fix

* few minor bugs fixed
* Stereogrid apply_func passes Vec3 instead numpy array
* Pair H method to get mutual rotation implemented
* velgrad method of DefGrad accepts steps kwarg
to generate list of DefGrad tensors
* Added Tensor class to work with deformation tensors

* Stereogrid always use Euclidean norms as weights
* DefGrad properties e1, e2, e3 (natural principal strains) added
* DefGrad properties eoct, goct (octahedral strains) added
* DefGrad from_ratios class method added
* DefGrad properties k, d, K, D (strain symmetries and intesities) added
* New class Ellipsoid added to work with ellipsoids
* FabricPLot renamed to VollmerPlot for consistency
* RamsayPlot, FlinnPlot and HsuPlot implemented
* All fabric plots have new path method accepting list of tensors

* Stress always gives eigenvalues sorted
* Stress I1, I2, I3 properties for invariants implemented
* Stress mean_stress property implemented
* Stress hydrostatic and deviatoric properties implemented
* precision added to settings to control numerical comparisms
* figsize added to settings to control figure size across APSG
* Animation examples fixed
* rand class method implemented for Fol, Lin, Vec3 and Pair to
generate random instance
* Group to_csv and from_csv improved
* SDB tags method works properly for multiple tags
* SDB can modify database metadata
* QGIS 3 plugin ReadSDB compatibility