Fix for issue #585

This PR provides a fix for the issue described in https://github.com/ioam/topographica/issues/585.
The issue is related to the match conditions, and the problematic lines are the following:
```for incoming_key, incoming_value in matchconditions.items():
             if incoming_key in src_sheet.properties and \
                    str(src_sheet.properties[incoming_key]) not in str(incoming_value):
                 matches=False
                 break
```
There are two issues with that. For easier description, see the following matchcondition:
```
{'level': 'LGN', 'polarity':properties['polarity'],
                 'opponent':properties['opponent'],
                 'surround':properties['surround']}
```
What we want to achieve is that this sheet connects to all sheets which have the following properties:
a) level is LGN AND
b) polarity is the same as the dest sheet AND
c) opponent is the same as the dest sheet AND
d) surround is the same as the dest sheet

Using the code above, there will always be a match, except one of the conditions is false. But when is it false? Only if the src sheet also has the key in its properties and the corresponding value to this is not contained in the property value of the dest sheet. Both very vague assumptions, which needed clarification.
If the key is not in the properties of the src sheet, there should not be a match (here, it caused a color sheet to be matched with a SF sheet, because c) and d) were skipped and a) and b) matched). Furthermore, there should be an exact match between the property values of src and dest sheet.
Historically, I was not able to make it an exact match, as I was using the old color model with separate retinas. The non-existing check whether the key is contained in the properties in the source sheet always was a bug, but never seemed to cause issues.

The same mistake was made in topo/submodel/specifications.py, where startswith was used rather than an exact match. There, it was only used for showing a summary of the model, but still it caused confusion in more complex models.

Finally, two separate matchconditions were merged for the color model.