[bugfix] Implement proper copy of BSplineLFE
Since the local basis stores a reference to the LFE,
we must implement a copy constructor to avoid a dangling
reerence. Due to guaranteed copy elision there is no
copy when storing the BSplineNode
in the LocalView
.
Hence this problem does not show up in practice.
The situation is different when using a power of
a BSplineBasis
. Then the PowerNode
stores several
copies. Hence accessing the basis fails with a segfault
due to the dangling reference.
This is the reason why the extended basis check failed
with a segfault for the BSplineBasis
.