Somes fixes to TelemacFile

Fix #983 (closed), which actually references two bugs:

• TelemacFile.nplan now returns 0 for 2D meshes, as in Fortran. The previous behaviour caused issues when TelemacFile was used without Hermes, as nplan was set to 1 instead of 0 when a SELAFIN file was written.
• TelemacFile now correctly writes the ipobo array whether or not a boundary conditions file is present.