Fsgrid testing and restructuring

[cscjlan]

This updates vlasiator to use the updated FsGrid. Most changes are due to:

• automatic code formatting by clang-format
• FsGrid member variables becoming private: accessing them through a function (mostly DX, DY, DZ)
• FsGrid member variables becoming const: returned variables must be const ref instead of ref
• FsGrid construction happens in one go, not in three steps: added some input parameters
• FsGrid is not copyable anymore: filterMoments copies and updates the data of a grid object instead of the object itself