Multiple BoM files in the spreadsheet

[hildogjr]

A interesting feature to add is generate a spreadsheet BoM using multiple xml BoM file. Because some time one project have more than one board to be made or purchase more the one may reduce importation taxes.

[hildogjr]

Still need improvements to work fine, but this have the general concept to the rest of the contributors help.

[hildogjr]

Next step: improvement to different board quantities.
Using the manf#_subqty field (maybe change it to just manf#_qty) that is filled in the read files routine with the number of quantity argument (1 : / 2 : ).
At the read file loop in kicost.py could be added a *{p1}, *{p2} , ... for the file order.
And in the subpart_qty() on eda_tools.py this {pX}be replaced with different cells references.