Prevent inadvertent attribute assignment in Quantity

[speth]

Changes proposed in this pull request

• Add a 'slots' definition to the Quantity class so trying to set the state using a property that doesn't exist for a particular
  phase model (e.g. HPQ) will not fail silently

If applicable, fill in the issue number this pull request is fixing

Fixes #1093

If applicable, provide an example illustrating new features this pull request is introducing

gas = ct.Solution("h2o2.yaml")
q1 = ct.Quantity(gas, mass=1)
q1.HPQ = gas.H, gas.P, 1  # raises AttributeError

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• The pull request is ready for review

[ischoegl]

Looks good to me / verified that this works. But I haven't used this feature before myself, so it's probably a good idea if @bryanwweber would have a look also.

[bryanwweber]

I likewise haven't used these features but it also LGTM.

[ischoegl]

I guess someone needs to approve ...