Change default directory for dump/restore.

[p-snft]

Extracted from oemof/oemof-solph#1027:

There was a problem because of old dump file. Writing a new one failed for whatever reason. So, energysystem.restore() restored whatever results were already present instead of newly generated results.

I don't think that the default behavior is sensible. Why would I dump the energy system into a .oemof folder in the $Home directory? Wouldn't it make more sense to use the current working directory (os.getcwd()) or the directory of the file from which the energysystem.dump()/energysystem.restore() command was called?

Originally posted by @lensum in oemof/oemof-solph#1027 (comment)

[pieterhexen]

Instead of splitting the file path into dpath and filename can there be a single argument for energysystem.dump()/energysystem.restore() like file.

Imo this would be more in line with save/dump methods of other libraries and facilitate the usage in workflow tools.