[BUG] new versions of old labs

[Tevian Dray]

I have revised several labs based on my teaching experiences this term.  The
current set is available by following the "Group Activities" link on the
"Labs" page on the website; the previous set have been moved to the "Labs used
during 2004" link.

Here are the changes:

HILL:
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* Included the topographic map, rather than making the students draw it.

VALLEY:
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* Included the topo map.
* Inserted the correct units.
* Removed the step-by-step directions.

WIRE:
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* Expressed the vector field in both rectangular and polar bases.
* Removed preliminary questions (sketch the field and find dr vector).

While I was at it, I also made minor changes to the "Acceleration" and
"Finding dr" labs, although I personally incorporate these into my lectures,
rather than doing them as group activities.  Finally, I took out the "Finding
Potentials" activity, as this is really a quick review of the Murder Mystery
Method rather than a good group activity.

Comments welcome.

Tevian