Release v0.6.2

Release v0.6.2

Release v0.6.1

Bug fix and minor feature additions.

v0.6.0

Release v0.6.0

Release v0.5.0

This is a checkpoint release before a major overhaul.

Release v0.4.0

Now pairwise potential functors support basic arithmetics:

```
md::lennard_jones_potential lj;
md::harmonic_potential harmonic;
auto weighted_sum = 0.3 * lj + 0.7 * harmonic;
```

Release v0.3.1

Fix include-all header.

Release v0.3.0

This release features a support for simple periodic boundary conditions
in computing short-range interactions.

Release v0.2.2

Extra `system::view` performance improvement.

Release v0.2.1

`system::view(key)` is now almost zero-overhead. Some simulation using
custom forcefield would benefit from this change.

v0.2.0

Intra- and inter-subsystem forcefields