CSVFileStore: reading in scenario data with missing data raises error

[willu47]

Given a csv file containing scenario data with one or more dimensions which are correctly specified in the file. If the corresponding data is missing, an error is raised when reading the file.

For example, demo_dimension has coordinates A,B,C, timestep has elements 2015, 2020.

This will work fine:

timestep,demo_dimension,values
2015,A,0
2015,B,0
2015,C,0
2020,A,0
2020,B,0
2020,C,0

This will fail:

timestep,demo_dimension,values
2015,A,0
2015,B,0
2015,C,
2020,A,0
2020,B,
2020,C,

Desired behaviour is to read in the data and fill missing data with NAN