Raise error for complex arguments to Coherent, DisplacedSqueezed and Dgate gates

[ziofil]

Adds warnings if Dgate, S2gate and BSgate use new default values and errors if the arguments are complex.

[codecov[bot]]

Codecov Report

Merging #428 into master will decrease coverage by 5.23%. The diff coverage is 96.65%.

@@            Coverage Diff             @@
##           master     #428      +/-   ##
==========================================
- Coverage   97.84%   92.61%   -5.24%     
==========================================
  Files          58       69      +11     
  Lines        6784     7096     +312     
==========================================
- Hits         6638     6572      -66     
- Misses        146      524     +378     

Impacted Files	Coverage Δ
strawberryfields/apps/plot.py	100.00% <ø> (ø)
strawberryfields/apps/train/embed.py	100.00% <ø> (ø)
...awberryfields/backends/gaussianbackend/__init__.py	100.00% <ø> (ø)
strawberryfields/backends/gaussianbackend/ops.py	100.00% <ø> (+2.70%)	:arrow_up:
strawberryfields/circuitspecs/circuit_specs.py	100.00% <ø> (ø)
strawberryfields/circuitspecs/gaussian_unitary.py	100.00% <ø> (ø)
strawberryfields/circuitspecs/tensorflow.py	100.00% <ø> (ø)
strawberryfields/backends/fockbackend/ops.py	53.57% <23.52%> (-38.43%)	:arrow_down:
strawberryfields/backends/fockbackend/circuit.py	69.23% <56.25%> (-29.20%)	:arrow_down:
strawberryfields/backends/fockbackend/backend.py	61.48% <58.33%> (-38.52%)	:arrow_down:
... and 54 more

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[antalszava]

• The mentioned 3 gates (Coherent, DisplacedSqueezed, Dgate) raise errors if any of the arguments are complex (no warnings)
• Tests were adjusted including checks for raising errors