TODO

* Parameterize a few things / couple with options:
  x SEFD
  - Default source brightness
  - Default filter parameters

* Spectrum model: 
  x shaped broadband noise
  x simple tone
  - Interpolate tone for more precision
  - PNR

* Doppler shift signal

* Consider horizon

* Add antenna-based defects
  - bandpass features
  x clock errors
  x packet loss / invalid packets

x Support complex-valued VDIF output

* Respect thread-id included in the format specifier

* Add polarization leakage

* Update config file format
  - Top level config
    - ?
  - Source ID (phase center number) sectioning
  - Antenna sectioning
    - Add antenna-based delay errors


STRETCH GOALS

* multi-phase-center -> source with structure

* SEFD depends on elevation


RESOLVED ISSUES

X Integrate with difxmessage
  - more integration still possible

X integrate with MPI
  - option 1: simply parallelize across .input files

X Create new DiFX repo for this: vdifsim

X Resolve phase/delay drift issue  (~10ns on long baselines)

X Add clock offsets

X get SEFD and flux density correct in _voltage_ units