Exporter prefix

[aancel]

Following the test written by @gdolle (testsuite/feelts/test_bdf3.cpp), we noticed some issues regarding the prefix used by the exporter:
An example would be (from @gdolle's code):

# First create an exporter with the test_bdf prefix
# This will create the initial timeset with the corresponding prefix
auto e = exporter( _mesh=mesh, _name="test_bdf" );
...
# Further calls to setPrefix won't modify the prefix of the timeset 
# after this initial call, because the setPrefix function does not update 
# the prefix of the latest timeset.
e->setPrefix( prefix )

Actually and currently, this is working with the Ensight Gold exporter because I made modifications to actually use the prefix set to the exporter and not the name of the timeset, but I think it would be a better idea to totally switch to using the name of the timeset.

The function setPrefix( name ) currently only store the value for the prefix. Wouldn't it be a better idea to directly set the value of the most recent timeset rather than storing the prefix ? Maybe we can also add a new setPrefix( ts_id, name ) that would allow to modify the prefix for other timesets (in case we would be using several of them)

[prudhomm]

I would have to have a closer look. I wrote that a long time ago. I need to refresh my memories.

[aancel]

I can take care of this and the associated modifications in the exporters, if you validate the change. I think it would make the code of the exporters cleaner.

[prudhomm]

sure, do that in a branch + PR

[prudhomm]

@aancel Please discuss here or the branch/PR the changes before hand rather than after.