Remove whitespace trimming of read XML content.
Removes a boost call. Could be written in C++11, but not sure of the rationale of removing the whitespace and it did not affect tests.
I recall issues when xdmf3 was being developed where whitespace in .xdmf files was breaking VTK level tests of XDMF. I don't know yet if this particular spot will actually matter in practice but it may and we'll find out when the VTK level tests of XDMF IO run.