Add ability to exclude attributes from processing results

[henhuy]

This issue is related to #707

Problem description:
When using oemof_tabulars facades energysystem components are generated with multiple attributes which are not related to oemof.solph. Using processing.parameter_as_dict() of oemof.solph results in many unwanted parameters from related component (i.e. subnodes as written in #707). It would be nice to have an option to exclude attributes from parameter dict generation.

Proposal:
Add attribute exclude_attrs=None to parameter_as_dict() which can hold a list of unwanted attributes, which are then filtered out before preparing parameter dict results. Default results are untouched as exclude_attrs is optional.