Add extra functionality to UOM

The object product_uom work using only one factor to calculate the conversion between UOM's, this works well for most cases, but I think it would be desirable to add a field to create a python function camera handle more complex cases.

For example:

To convert Celsius to Fahrenheit using the formula:

Temp. (F) = 1.8 * Temp. (° C) +32

This can not be defined by a "factor", but can be easily implemented in a python function.

Affects:
product_uom._compute_qty_obj

Note: Workflows do something like this to change document status