Add support for importing NetBox 3.3.x data

[glennmatthews]

With #92 as a prerequisite, the following NetBox data model changes may need to be accounted for in the importer. Changes in models already noted in prior issues as not relevant to Nautobot (such as WirelessLAN) are not included below for brevity.

• Change of IPAddress.nat_inside from a OneToOneField to a ForeignKey (allowing multiple nat_outside records) - a similar feature was added in Nautobot 1.3
• Addition of VirtualMachine.site and VirtualMachine.device fields - no Nautobot equivalent
• Change of VirtualMachine.cluster field to nullable - null values are not presently permitted in Nautobot, will need handling
• Addition of Circuit.termination_date field - no Nautobot equivalent
• Addition of RackGroupLocation.status and Cluster.status fields - no Nautobot equivalent
• Support for custom fields and tags on CircuitTermination - Nautobot has supported custom fields here since 1.0 and tags since 1.3.
• Removal of maximum length limits on CustomLinks - will need to be truncated on import to Nautobot
• Addition of ConfigContext.locations field - recall that NetBox's locations are what Nautobot calls rack groups, this is not the same as Nautobot's ConfigContext.locations field. No Nautobot equivalent.
• Addition of Token.allowed_ips and Token.last_used fields - no Nautobot equivalent
• Addition of CustomField.group_name - equivalent to Nautobot CustomField.grouping added in Nautobot 1.5
• Addition of CustomField.ui_visibility - similar to Nautobot CustomField.advanced_ui added in Nautobot 1.3
• Change of DeviceType.u_height and Device.position to support decimal values - will need to be coerced to integers for Nautobot, will need to think about how to handle adjacent devices spanning partial rack units though.
• Addition of Interface.poe_mode and Interface.poe_type - no Nautobot equivalent
• Addition of L2VPN and L2VPNTermination models - no Nautobot equivalent
• Major changes to the Cable, CablePath, and CableTermination models, and associated changes to all cable-able port models - this will likely be difficult to support at this time.
• Additional type enums (#9935 add new wireless choices for interfaces netbox-community/netbox#10116, Fixes: #10356 backplane connections netbox-community/netbox#10554, netbox-community/netbox@1e0b024, etc.)