Update S2gate convention to fit sf

[thisac]

Context:
The wrong convention was used to implement the two-mode squeezing gate (using a minus instead of a plus between the terms in the definition).

Description of the Change:
Changed the argument in the tanh-function for the two_mode_squeezing function in fock_gradients.py from -r to r (i.e. removing the -).

Benefits:
The Strawberry Fields tests now pass, due to the same conventions being used.

Possible Drawbacks:
None

Related GitHub Issues:

[nquesada]

All good from my end. This will require to update the sf docs which did not coincide with the implementation of S2.

[josh146]

Looks good from my end!

[codecov]

Codecov Report

Merging #130 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #130   +/-   ##
=======================================
  Coverage   97.61%   97.61%           
=======================================
  Files          12       12           
  Lines         839      839           
=======================================
  Hits          819      819           
  Misses         20       20           

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