Etabackend

1. recipe: added header to save txt in Correlation recipe
2. packaging: The version number is taken from the setup.py
3. gui: fix an issued caused by frontend timeout (138 )

1. **correlate timetag files from more than one time taggers** (99 )
2. replacing Acqusition Devices with the new RFILE Tool (117 )
3. eta.run supports auto-feed from multiple sources (125 ,122, 116 )
4. serve GUI from the backend
5. support Mac OS
6. use bokeh for realtime display (133)

**NOTE: Acqusition Devices in the GUI has been and replaced with the new RFILE Tool. Please [refer to the manual](https://eta.readthedocs.io/en/latest/python.html) to update your recipe.**


**NOTE: The built-in correlation recipe has roll-backed [the setting of bin ranges on left hand side](https://github.com/timetag/ETA/issues/59#issuecomment-599657842), graphs might looks different compared to 0.7.0. Customized correlation recipe will not be affected.**

1. the new Clip system and improved API set(107,110,113)
2. supporting streaming analysis and simulations
3. improved information display (114)
4. separate ETA and Backend to make ETA an independent callable library (120)
5. adding tests

**NOTE: This is a major update to both ETA GUI and Backend. Multiple APIs has been changed. Please [refer to the manual](https://eta.readthedocs.io/en/latest/python.html) to update your recipe. Feel free to report an issue if anything doesn't seem right.**

1. supporting Linux (96)
2. improve installation experience

1. fixing the problem that incremental_cut function uses the wrong item in the cut definition for ByteSize (89)
2. supports scientific notation for Tools in Instrument Designer (95)
3. solving an issue that gets the frontend stuck (94)
4. `wait_till_presnese` is renamed to `wait_for_data` (93)
5. updated correlation recipe to support HydraHarp T2 and T3 formats as well as quTAG format
6. implemented improved realtime script into Realtime.eta, Lifetime.eta
7. add cut_modifier_syncrate for override the syncrate from the timetag header

**NOTE: `wait_till_presnese` is renamed to `wait_for_data`, please update your existing recipes.**