Saving runtime data as list of dicts in datacontainer

Description

As pointed out in #282 (closed) data is not longer appended to a DataFrame but to a list of dicts during simulation.

As a consequence the following changes were implemented as well:

• runtime data is saved to a numpy format file. However dc.read function allow users to read files with the former format by using the flag old_format=True
• dc.get_data creates an internal DataFrame only with the requested columns which helps to keep the performance of this function.
• dc.data property returns a DataFrame which excludes occupations
• dc.observables returns the keys of the first element of the data list with the exception of 'mctrial'
• dc.append has been modified accordingly keeping the exceptions established in the previous version with the DataFrame
• Unittest and docstring updated

Closes #282 (closed)