Output time from a file

Currently, we are only taking dt for the output time. In cosmological simulation, a different choice is possible with the scale factor (e.g. using da). It would be nice to use a file that contains all the output time.

Here is a proposition of format

# scale_factor
0.1
0.2
0.3

# redshift
10.
2.
1.

# age
2.
5.
10.