i/o: improve adios2 field output and reader

[germasch]

Most importantly, the .bp files themselves should store more info, which should then be read and used when reading.

This requires:

• the names of all the components saved
• maybe the staggering, too, though acting upon that automatically is probably hard and maybe not really important
• Information on the global domain (dims, physical lengths); Should be used to recover coordinates. A choice of units wouldn't be bad, either ;)
• Information on the species in the simulation