Schemdraw

- Changed Header anchors to `pinN` for consistency with Ic.
- Improved label placement with respect to anchor positions.
- Prevent duplicate figures from showing in Jupyter Element representation
- Improvements for headless server operation to prevent popup window
- Added some undocumented features to documentation
- Added `Drawing.get_imagedata` function for returning raw image bytes
- Fixed pip installation issue with module capitalization. Must import lowercase `schemdraw`.

- Bug fix: restore usage outside of Jupyter, so that Matplotlib window is shown when calling Drawing.draw().

- Dropped support for Python 2. Now requires 3.7+.
- Elements are now subclasses of Element. Previous (dict) element names are translated into
new class names. Any user-defined elements will need to be converted to classes.
The `group_elements` function is replaced with `ElementDrawing` class.
- Allow `fontsize` or `size` keyword arguments interchangeably in Drawing and add_label
- Updated flow.Decision to use keyword arguments for labeling decision branches
- The Ic element label offset parameter changed from `lblofst` to `lofst`
to avoid conflict with the main element label.
- Direct access to Drawing.fig and Drawing.ax are no longer available. Instead, Drawing.draw()
returns a schemdraw.Figure instance with fig and ax attributes.
- Implemented Jupyter representation functions for both Drawing and Element classes.

- New Elements:
- Coax
- Triax
- SwitchDpst
- SwitchDpdt
- Relay
- Optocoupler
- Arrow
- LineDot
- Breaker
- OrthoLines
- RightLines
- BusConnect
- BusLine
- Tag
- Photoresistor
- PhotoresistorBox
- Thermistor
- DiodeShockley
- PotBox
- RBoxVar
- Solar
- Neon
- SourceSquare
- AntennaLoop
- AntennaLoop2
- AudioJack
- Tgate
- Schmitt
- SchmittNot
- SchmittAnd
- SchmittNand

- Refactored internals to allow more control over individual components of drawing.
Should have no effect unless the user is accessing internal attributes of the Element object.
This also adds the `segments` list to the Element object, which allows finer control over
individual bits of the drawing.
- Updated add_label so that "top" labels should always appear on top, regardless of flip/reverse
- Swapped the direction of current sources, so that a current source with direction "up" has the
arrow pointing up.
- Added "zorder" parameter in the element definition dictionary and `add` method
- Added `elements.ic` and `elements.multiplexer` functions as replacements to `blackbox` and `mux`.
These include more functionality such as adjusting indiviudal pin rotation, color, and inverter bubbles.
- Labels can be placed relative to an anchor position using the `add_label` method. This could be
useful, for example, in labeling pin numbers on a logic gate or opamp.
- Added new anchors to OPAMPs for power supply and offset nulls.

- New Elements:
- MIC
- MOTOR

- Documentation:
- Upgraded documentation to Sphinx and moved to readthedocs.org at https://schemdraw.readthedocs.io/en/latest/.
- Changed preferred import to `import SchemDraw.elements as elm`. Apparently some people still
use import * with pylab; this suggestion will help avoid conflicts.

- Added flowcharting symbol methods to SchemDraw.flow module
- Added signal processing symbols to SchemDraw.dsp module
- Implemented fill parameter on Drawing.add to fill shapes and closed paths with a solid color
- New elements:
- Fuse
- CapacitorVar,
- DiodeTunnel
- Jfet
- Diac
- Triac
- SCR

- Fixed drawing of NOT and related gates to property extend the path
- Fixed arrow translation when grouping elements
- Fixed sidelabels and plabels of blackbox when empty
- Fixed arc drawing due to change in Matplotlib 2.2 on asymmetric partial arcs