Stix2

* 86 Adds helper function to Location objects to generate a URL to the location in an online map engine.

* 234 Update documentation structure to better navigate between v20/v21 objects
* 232 FileSystemStore now raises an exception if you attempt to overwrite an existing file
* 236 Fix a serialization problem with the WindowsRegistryKey observable object
* 238 Fix a problem with the LanguageContent object not allowing its creation with an empty dictionary

Most (if not all) STIX 2.1 SDOs/SROs and core objects have been implemented according to the latest CSD/WD document
There is an implementation for the conversion scales
196, 193 Removing duplicate code for: properties, registering objects, parsing objects, custom objects
80, 197 Most (if not all) tests created for v20 are also implemented for v21
189 Added extra checks for the pre-commit tool
202 It is now possible to pass a Bundle into add() method in Memory datastores

* 225 MemorySource fix to support custom objects
* 212 More consistency in Observable extensions behavior/error messages

* 187 Pickle proof objects
* 181 Improvements to error handling for datastores
* 184, 192 Fix "pretty" JSON serialization problems
* 195 Fix wrong property name for raster-image-ext
* 201 UUIDv4 enforcement on identifiers
* 203 New filter option "contains" for datastores
* 207 Add documentation on patterning
* 213 Python 3.7 support
* 214 Support for multiple object versions in MemoryStore

* Fixes bugs when using allow_custom (176, 179).