fix tolerance in test-fe for comparison of coefficients.

While trying to adapt the interpolation functions for SIMD datatypes (!213 (closed)) I stumbled over the problem that the test-pk2d test failed. The reason was that the tolerance used to check whether the coefficients are correctly reproduced in testInterpolation is actually to small. You can try it by yourself changing the number of runs n in line 86 in test-fe.hh to 50. The test test-pk2d will (probably) fail then.

As the FEFunction computes a linear combination with coeff.size() summands I propose to increase the tolerance for the check by a factor of 2*coeff.size(). I don't ran into problems then.