Address error C2512 when compiling with Visual Studio

[ischoegl]

Please fill in the issue number this pull request is fixing:

Fixes #670

Changes proposed in this pull request:

• Error is avoided by explicitly passing the default argument to the constructor, i.e. UnitSytem U({}); instead of UnitSystem U;.
• The issue appears to be that Visual Studio erroneously doesn't recognize the optional argument. See Visual Studio: Compiler Error C2512

[codecov]

Codecov Report

Merging #676 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #676      +/-   ##
==========================================
+ Coverage   70.62%   70.62%   +<.01%     
==========================================
  Files         372      372              
  Lines       43564    43565       +1     
==========================================
+ Hits        30768    30769       +1     
  Misses      12796    12796

Impacted Files	Coverage Δ
include/cantera/base/Units.h	100% <100%> (ø)	⬆️
src/transport/GasTransport.cpp	90.58% <0%> (ø)	⬆️

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[speth]

I agree that the error seems to be in VS2019 (especially as earlier versions mostly work, although there was a related issue in VS2017 that was fixed in 8502d18).

I'm wondering whether the better fix here is to explicitly implement a default constructor and remove the default argument from the existing constructor, with a note that this is done as a workaround for an issue with VS2019. I suggest this both because for any future use of the UnitSystem class, (a) the appropriate fix is not immediately obvious due to the error message itself being incorrect, since the class does have a valid default constructor, and (b) the declaration that should work (UnitSystem U;) will work fine on all other compilers/platforms and only be discovered when tested specifically with VS2019 which means that this same issue could easily pop up again.

[ischoegl]

Valid point, especially as a permanent fix is trivial. Done.

[speth]

Good use of a C++11 delegating constructor.