Adopt PEP-8 for whitespace recommendations

[ischoegl]

Abstract

The current style guide for whitespace (per CONTRIBUTING.md) recommends:

• Use whitespaces to improve code readability (examples: after commas; before and
  after mathematical operators (+/-/*// except ^), binary operators
  (&&/||/...), and comparisons (</>/==/...); before and after equality

does not follow PEP-8 recommendations, i.e.

# Correct:
i = i + 1
submitted += 1
x = x*2 - 1
hypot2 = x*x + y*y
c = (a+b) * (a-b)

and

# Wrong:
i=i+1
submitted +=1
x = x * 2 - 1  # <-- current Cantera style 
hypot2 = x * x + y * y  # <-- current Cantera style 
c = (a + b) * (a - b)  # <-- current Cantera style 

as well as some other discrepancies.

Motivation

The PEP-8 recommendation seeks to improve code readability, so it should be adopted for all coding languages supported by Cantera.

Possible Solutions

Update style recommendations in CONTRIBUTING.md, and use style going forward. Add comment that aggressive substitutions are not encouraged.

References

Python PEP-8 Whitespace in expressions and statements