Improve input unit flexibility in TEDFs #15

Some sources for the processes Methanol to Olefins and Naphtha Steam Cracking report functional units from simulations that can be given as kg/s instead of kg, for example. This is then reported relative to the product output, also in kg/s. However, unit_convert does not support such entries, raising that kg/s cannot be converted to kg.
Additionally, such unit conversion issues are not caught with Exceptions, making it hard to identify the row affected.

Suggested Fixes:

• add another step in unit conversion, which first simplifies the reported units for unit and reference_unit relative to one another (ie. here, convert kg/s to kg for both entries).
• add Exceptions that can catch such unit conversion issues