Use this pointer for subPreBasis(i) in lambdas [bugfix]
Adds the pointer
this-> when calling
subPreBasis(i) inside of lambdas.
The recently introduced function
subPreBasis(i) that returns the i'th subPreBasis is used in lambdas without the
this-> pointer. This is fine in recent compilers, but fails in gcc6.
Maybe one should add a test for older compilers in the ci system, to catch such bugs.