[IMP] web: many2one as measure in graph & pivot only when desirable

With this commit, the fields many2one are not always measures in pivot
and graph views. By default, they are *not* measures.

In order to have them as measures, we should pass `additionalMeasures`
as a param to any of the views, which contains a list of fields to consider
as measures.

example:

    Suppose a model with the following fields:

        - `product_id` (many2one)
        - `categ_id` (many2one)
        - `sold` (float)

    By default, only the field `sold` is a measure
    in a graph or pivot view.

    In order to pass `product_id` as a measure of
    a graph view, you should instantiate this view
    as follow:

``
    var viewInfo = /* ... */;
    var params = {
        additionalMeasures = ['product_id'],
    };
    var graph = new GraphView(viewInfo, params);
``

An exception to the rule above is when the many2one field is of type `measure`
in the arch: this field is set as the default measure of the view, therefore
it is implicitly added as an additional measure.