Better control over reading input start file with XIOS

Note that if one has

xios_output = false

in run.def , when loading the start file the following error will be generated:

In file "field_impl.hpp", function "void xios::CField::getData(xios::CArray &) const [with int N = 3]",  line 50 -> Impossible to access field data, the field [ id = ps_start ] does not have read access.

Even though the start file is there and contains the field... It seems one must have

xios_output = true

So perhaps this flag should be considered obsolete and removed? Or some more control and explicit error message be given to the user?