Config sensitive to field-ordering

[benjimin]

These two config files differ only in the list-ordering, but produce different outcomes.

This works:

- template: wind_v3
- load_exposure:
    exposure_latitude: LATITUDE
    exposure_longitude: LONGITUDE
    file_name: .../exposure.csv
- load_wind_ascii: NetCDF:.../gust.nc:vmax
- calc_struct_loss:
    replacement_value_label: REPLACEMENT_VALUE
- save: .../test_impact.csv

This fails:

- calc_struct_loss:
    replacement_value_label: REPLACEMENT_VALUE
- load_wind_ascii: NetCDF:.../gust.nc:vmax
- save: .../test_impact.csv
- template: wind_v3
- load_exposure:
    exposure_latitude: LATITUDE
    exposure_longitude: LONGITUDE
    file_name: .../exposure.csv

With uninformative message (from config_build.py, called from templates.py, called from config.py):

RuntimeError: 
Invalid key in config file; calc_struct_loss 

Possibly the template must be specified first, to control how subsequent fields are parsed. This is either a bug or should be clearly documented. (At minimum, since the config is the primary user interface, the error message should be more helpful.)