If there are two speed-contours and they do not intersect, check
whether outermost contours intersect. The intersection threshold for
outermost contour is defined by new variable `min_inters_out`. Note
that the default value is such that results are not changed by this
commit if `min_inters_out` is left to its default value.

Because we are going to introduce another variable for the
intersection of outermost contours.

I do not remember why I chose to write to standard error. First it is
clearer to write only error messages to standard error. Second the
order of messages on standard error coming from the Fortran code and
from the shapelib C library does not seem deterministic, and this is
annoying to compare runs.

Because we want to call `config_graph` in sequential program
`test_overlap`.

Change suffix because we are going to add preprocessor directives.

Which read namelists. Motivations: diminish the number of arguments of
procedure overlap; lighten the main program unit. We conditionally
compile MPI calls in `read_grid` because we want to use variables of
module`read_grid_m` in procedure `read_snapshot`, and `read_snapshot`
is called from sequential program `test_nearby_extr`.