`map_to_points` for `segments`

[pescap]

While working on #218, I noticed that map_to_points exhibits an issue with restrictions on segments (multishape):

import bempp.api
grid = bempp.api.shapes.multitrace_cube(h=1)
segments = [1, 2, 3, 4, 5, 6]

P1 = bempp.api.function_space(grid, "P", 1, segments=segments)
P1.map_to_points()

This script gives: ValueError: cannot assign slice from input of different size

The same error occurs for example for (DP,0), (RWG,0).

[pescap]

Sounds to be related to:

bempp-cl/bempp/api/fmm/helpers.py

Line 466 in c673ea8

for elem in support_elements:

When using segments, elem is no longer equal to 0,1,2,3 ....

[pescap]

I started working on it: #221. It resolves the error, but it doesn’t seem sufficient to have the FMM work properly for spaces with segments.