Allow phase look-up in Mixtures by both name and phase id

### Cantera version
all

### Operating System
all

### Python/MATLAB version
Python 

### Expected Behavior
The index of a phase ('Solution') within mixtures can be identified either by `name` or `ID`
(using the `Mixture.phase_index` method)

### Actual Behavior
`Mixture.phase_index` currently requires `name`.