Create on demand packs of products for POS

Allows the creation of products as Packs that can contain products with
variants.

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Building products :
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- Create products using product_variant_multi (from Akretion -
http://www.akretion.com), meaning using variants features

Building a Pack :
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- Create a product as usual and name it, say Pack1
- Check Custom Pack checkbox
- Type of product should be set to Service automatically
- Go on Pack tab
- Add templates products grouped by index :  each group of templates
will be displayed in a list in POS
- That's all

Packs are avaiable in POS :
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Packs are avaiable in POS as usual products (visualy).

When you select a pack, a new screen is displayed and you can select one
template for each group, the select the needed variant you set
previously.

Pack is added to caddie as well as its products, but thoses products
prices are set to 0.00 as well as on receipt.

This way, you can set a reduced price for the pack , even if the sumn of
each products may be superior. (This is for what packs are made),
but also, you can build your pack on-demand, by making groups of
templates .