Spacepackets

- Small bugfix for PUS 1 Step ID unpacking

- Added `PusVerificator` module which can track the verification status of sent telecommands
- Added several magic method implementations, notably `__eq__` and `__hash__` where
applicable
- Removed PUS A support completely. PUS A is relatively old, and specialicing on one packet version
makes the code a lot simpler
- Added `ecss.fields` module which contains the `Ptc` and various PFC enumerations. Also add
a generic abstraction for enumerated fields in form of a `PacketFieldEnum` and a
`PacketFieldBase`. This is useful to have an abstraction for the various PUS standard packet
fields which can have variable sizes

- Minor name change for PUS 17 and PUS 1 TM classes
- New `RequestId` class to encapsulate the field used by the PUS 1 Verification
service
- Update CRC16 handling for TMTC PUS classes. It is possible to calculate the CRC16
manually with a dedicated `calc_crc` call and then omit the calculation in
the `pack` call with an additional argument.

- New Helper objects for CCSDS Space Packet subfields, namely new
`PacketId` and `PacketSeqCtrl` class
- Remove PUS A support for PUS telecommands
- Added multiple `__str__` and `__repr__` implementations where
applicable
- API simplification, shorter or better keywords for PUS TM and PUS TC
constructor calls

- API improvements, bugfix and general improvements for CCSDS spacepacket
header implementation

- Named value for fetching global APID