Allow relative file paths in ddcks

[ahobeost]

As a user, I would like to use the provided profiles (weather, demand, etc.) close to the ddcks that read them. At the moment, the path in the ddck can only be absolute or relative to the final .dck location.

It would be great, if pytrnsys could take the path relative to the ddck and adjust it to be relative to the final .dck location.

Example:

• structure
  ddck/weather/weather_reader.ddck
  ddck/weather/weather_profile.csv
  results/simulation.dck
• relative path in weather_reader.ddck
  assign weather_profile.csv :Unit
• output in simulation.dck
  assign ../ddck/weather/weather_profile.csv weather_readerUnit

[ahobeost]

After some further investigation, this issue is less severe as first thought.
In the .ddck one can use PATH$ to make pytrnsys use the absolute path of the ddck.

assign PATH$/weather_profile.csv :Unit

will become:
assign c:\path-to-ddck\weather_profile.csv weather_readerUnit

However, the original idea of getting paths relative to the .dck is currently not implemented.