Big cosmo volume

Following issue #30, I have added a new script to generate arbitrarily large initial conditions.

The scripts downloads an initial tile and then replicates it along each axis multiple times and/or downsamples the initial particle load. The script takes two arguments:

python makeIC.py factor copies

where factor is the downsampling factor [0, 1] and copies is an integer giving the number of copies along each axis.

See merge request !20


Former-commit-id: 21a0e5d16d4324566e3331f92f9c04ec00b38a67