"Typical" measurement

From Tim:

The "typical" measurement is defined as the single measurement done at all "typical" conditions, where a "typical" condition is given, and otherwise the average over conditions, if a "typical" condition is not given.

Enumerating over, say, ss, sf, tt, fs, ff, then tt is clearly the typical condition and should be used in calculating the typical value, but it won't be if you use "enumerate".

The following should be valid:

```
conditions:
corner:
enumerate: [ss, sf, tt, fs, ff]
typical: tt
```

Start by tracing how the configuration handles conditions.corner, especially enumerate and typical. Verify the current calculation for an enumeration such as [ss, sf, tt, fs, ff], then implement the requested behavior so tt is selected as the typical measurement; done means the shown configuration is accepted and uses tt.

Written by the indexing model from the issue text.