add build-requirements with pyproject.tml (no prior manual install of numpy & pybind11)

[FelixKleineBoesing]

• installing george via pip is now possible without prior installation of numpy and pybind11
• pybind11 v2.4.3 doesn´t support this build possibilty since the function get_include always points to the location where the header files are located when it´s installed. Since the package is located in a temporary build directory during the build process, the header files couldn´t be found. The recent master version of pybind11 has a new get_include function that is similar to numpy´s version and enables build requirements. Nevertheless, after the build process, the released 2.4.3 pybind11 version will be used.
• for now a specific master commit is referenced inside pyproject.toml, but that should be changed to the next tagged version. tests are all working under macos and ubuntu (dockerfile is added to root directory)

[coveralls]

Coverage remained the same at 85.486% when pulling db6fc25 on FelixKleineBoesing:feature/build-requirements into 4481968 on dfm:master.

[dfm]

Thanks for this! Can you send a link to the discussion of this feature on the pybind11 repo so we can keep an eye on it?

[FelixKleineBoesing]

Hi Dan,
this is the discussion to the mentioned changes in pybind11: pybind/pybind11#1995

[FelixKleineBoesing]

This problem is fixed with version v2.5.0 of pybind11.

[dfm]

I've finally fixed this in #140. Thanks!