assume each checkpoint file should be at least order 1GB, and then check
how much space each checkpoint will require.
this includes both field and particle data.

this means i defined a virtual "step" method, as well as "do_stats" and
"write_checkpoint".
I think this makes it even more clear what job is assigned to which
class.

Feature/longlong particles

See merge request !11

Ensure to find all neighbors processes (especially with width interpolator)

See merge request !10

Add simplified wrapper called `DNS`.
While this will make it slightly more complicated to generate
peculiar custom codes, the experience so far has been that this current
version is quite adequate for our needs.

Most of the code generation is removed from the Python wrapper, and
plain old C++ classes are written for each type of simulation that is to
be run.

Feature/limited particles output

See merge request !9

It also successfully gives the "wrong number of command line arguments"
message.

I need to figure out how to define the read_parameters method properly

Update particles output to limite the number of processes involved -- manage a minimum chunk size -- write a multiple of a chunk

Update bfps timer (debug)

See merge request !8

Update bfps timer -- std default duration constructor is not setting cumulated time to zero, so do it manually