ajdawson/windspharm: Version 1.7.0

The v1.7.0 release makes further progress on the road to more modern tooling and processes. The significant changes for users/contributors are:

• Support for using Legendre functions computed on-the-fly or stored, implemented by @rcomer [#97, #98].
• The source code directories have been reorganised, the lib/windspharm directory has been moved to windspharm/ (lib/ is removed) and the doc/ directory has been renamed docs/ [#105].
• The package version is now controlled by versioneer.

In addition, this is the first release where documentation and PyPI packages will be built automatically as part of the continuous integration system.