Export initial condition to VTX

[RemDelaporteMathurin]

It may be useful to streamline initial condition export for users.

I would like maybe something like this

my_model.initialise()

my_model.export_initial_condition(species=H, filename="initial_condition_H.bp")

my_model.run()

What do you think @jhdark ?

[jhdark]

Could indeed be useful