Plot accounts for specific products

the plot_accounts method now also takes a parameter which plot the
accounts for a specific product

btw: deleted a left over report_per_capita
moved exiobase parser in specific parser module
Global variables and constants are separated in a specific file
Change relative file path for the mrio data
bugfix outsourced parser - relative path work now
finished parser for single exiobase extension
status before restructuring
restructured whole project for consistent imports
changed to BSD 3 license
corrected notebooks
changes returns to yield in docstring for generators
finished new doc, some docstring corrections