Add default gvectors

[nedtaylor]

Set up in initialiser a default form for the total gvectors based on the covalent (and ionic???) radii of element pairs.

According to Joe, the angular terms should just be a flat 1.

[nedtaylor]

This would have an issue of multispecies pairs would have no initialisation.

[nedtaylor]

Going to ignore this for now.

[nedtaylor]

Fixed in merge #7.

Only considers nearest neighbour bonding and does not consider order. It uses combination of covalent radii. The short order is only considered because nothing can be suggested about long-range interactions.