Bugfix for periodic boundary conditions

[adediego]

Currently, the following code throws an exception:

using CoherentStructures
using Distances

LL, UR = (0., 0.), (1., 1.)
gs = 10
ctx, _ = regularTriangularGrid((gs, gs), LL, UR);
predicate = (p1, p2) -> peuclidean(p1, p2, [1.0,Inf]) < 2e-10
bdata = BoundaryData(ctx, predicate, [])

adaptiveTOCollocationStiffnessMatrix(ctx, identity; on_cylinder=true, bdata)

It looks like this happens because in adaptiveTOCollocationStiffnessMatrix there is an attempt to avoid unnecessary evaluations of the flow by only applying it to the first num_real_points points. The actual redundant points however, may be distributed differently across the indices. Also, in adaptiveTOFutureGrid, it is assumed that flow_map_images is populated with the images of every point, so this is where the error is thrown.

This seems like the most direct fix. As the redundant points are typically on the boundary and thus very few, the effect on performance should be negligible.