Due to the _set_product_template_field function, it is not possible
to set the standard price on a product template with more than one variant.
Due to the _compute_product_template_field function, it is not possible
to read the standard price on a product with more than one variant.
The field "standard_price" is in readonly to avoid some problems if the costing
method is in real time.
Closes: #11308
opw:671013
Goffin Simon
083e172dd9