Item overview load incorrect platform variants

[Nicklas Nordborg]

The item overview can sometimes load incorrect platform variant information. To reproduce this:

• Create a new platform with two variants
• Create one array design using one of the variants
• Create one raw bioassay with the other variant, and link it to the array design

Then, go to the overview tab for the raw bioassay. It will display the correct platform/variant for the raw bioasay, but for the array design it will include both variants.

One could argue that using different variants for the array design and raw bioassy is an error, but if they have the same variant, it will be listed twice for the array design.

[Nicklas Nordborg]

(In [6516]) Fixes #1833: Item overview load incorrect platform variants

This was a problem with caching a platform even if the variants was different, and then always adding a "variant" subnode, not checking if it already existed or not.