Fix SolutionArray extra slice

[ischoegl]

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

Closes #1203

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

In [1]: import cantera as ct
   ...: import numpy as np
   ...: 
   ...: gas = ct.Solution("gri30.yaml")
   ...: 
   ...: T = np.arange(900, 1000, 50)
   ...: estimated_ignition_delay_times = np.ones_like(T) * 0.005
   ...: ignition_delays = ct.SolutionArray(
   ...:     gas, shape=T.shape, extra={"tau": estimated_ignition_delay_times}
   ...: )
   ...: ignition_delays.TP = T, 40.0 * 101325.0
   ...: for state in ignition_delays:
   ...:     print(state.tau)
0.005
0.005

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]

@bryanwweber … this fix for the issue you reported is ready for review (and should be quick).

[bryanwweber]

Thanks @ischoegl, one case that I think needs to be investigated here.

As a total aside, outside the scope of this PR, but just to write it down somewhere, it'd be nice if SolutionArray supported:

1. len()
2. .shape
3. Accessing extra keys that can't be accessed by attributed because they have an invalid character (-, , etc.)

[bryanwweber]

Thanks! Once the tests pass this is good for me

[bryanwweber]

It seems like this should raise an IndexError, but insofar as this maintains current behavior, it seems fine to me to keep it like this and defer adding the IndexError.