Force InputFld to update fielddef

This forces InputFld to update fielddef after loading the expansions, instead of only doing this when range is specified. This is necessary because the number of partitions from the original file might not be the same as the number of processes being used in FieldConvert, causing the fielddef obtained from Import to contain extra elements.

For example, when converting a serial fld to a parallel format using
```
mpirun -np 8 FieldConvert file.xml file_serial.fld file_par.fld
```
the current implementation causes all 8 partition files from file_par.fld to contain the whole field.

See merge request !611