Apsg

* Group method centered improved
* Group method halfspace added to reorient all vectors towards resultant
halfspace

* Stress tensor with few basic methods implemented
* StereoGrid keyword argument 'weighted' to control weighting
* StereoNet kwargs are passed to underlying methods for immediate plots
* StereoNet tensor method implemented (draw eigenlins or fols based on
fol_plot settings)
* Group totvar property and dot and proj methods implemented
* Fol and Lin dot method returns absolute value of dot product
* Vec3 H method to get mutual rotation implemented
* StereoNet.contourf method draw contour lines as well by default. Option
clines controls it.
* centered bug fixed
* StereoNet allows simple animations. Add `animate=True` kwarg to plotting
method and finally call StereoNet animate method.

* Density class renamed to StereoGrid
* Fault sense under rotation fixed
* FaultSet example provided
* Angelier-Mechler dihedra method implemented for FaultSet
* StereoNet accepts StereoGrid and Ortensor as quick plot arguments
* StereoNet instance has axtitle method to put text below stereonet

* conda build for all platforms
* numpy, matplotlib and other helpres imported by default
* ortensor is normed by default
* ortensor MADp, MADo, MAD and kind properties added

* shell script iapsg opens interactive console

* Simple settings interface implemented in in apsg.core.seetings dictionary.
To change settings use:

from apsg.core import settings
setting['name']=value

* `notation` setting with values `dd` or `rhr` control how azimuth argument of
Fol is represented.
* `vec2dd` setting with values `True` or `False` control how `Vec3` is
represented.
* Vec3 could be instantiated by one arument (vector like), 2 arguments
(azimuth, inclination) or 3 arguments (azimuth, inclination, magnitude).
* Group and FaultSet can return array or list of user-defined attributes of
all elements