Physbo

This change makes it clear that code importing PHYSBO will not be affected by copyleft. However, if you modify and redistribute PHYSBO itself, copyleft will still apply, and you will need to redistribute it under MPL v2.
これにより、PHYSBO を import するコードについては、コピーレフトの影響を受けないことが明確になりました。一方、PHYSBO そのものを修正して再頒布する場合には、これまでと同様にコピーレフトが働き、MPL v2 の下で再頒布する必要があります。

What's Changed
* Change License to MPLv2 (https://github.com/issp-center-dev/PHYSBO/pull/55)
* Set the upper bound of numpy version (https://github.com/issp-center-dev/PHYSBO/pull/56)

**Full Changelog**: https://github.com/issp-center-dev/PHYSBO/compare/v1.1.1...v2.0.0

Release notes

Release note
Changes from v0.3.x
New features
- `policy.get_post_fmean(xs)` (17)
- mean values of the trained predictor (the post-distribution of the Guassian process) at points `xs`
- `policy.get_post_fcov(xs)` (17)
- covariance of the trained predictor (the post-distribution of the Guassian process) at points `xs`
- `policy.get_score(mode, xs_or_actions)` (16)
- score (acquisition function) at points xs or actions

Changes
- Move `physbo.search.discrete.policy_mo` to `physbo.search.discrete_multi.policy` (13)

Fixes
- Return best actions as an array of integer (12)
- Fixed a bug of crashing if no actions remain (14)

Documents
- English manual is uploaded.