Allow simple default uniform fuel layer

[emanuelegissi]

From Randy:
@emanuelegissi I am not sure if I have ever tried to use qgis2fds with a simple uniform fuel layer. But I think it would be nice to have this option, if it does not already exist. There could just be an option for a "custom fuel" or something that would provide the mapping from whatever SURF_ID to the SURF index in the FACE list. We don't want the default to just be INERT, because that will give a value of 0 for the SURF INDEX which would then have to be changed in FACE list if we wanted a different custom fuel in FDS. Hope this makes sense.

[emanuelegissi]

This is already ok with OBSTs. I need to update GEOMs to the same behaviour