When creating the ghost tasks as well as the hydro-related subgrid tasks, only do so if the cell actually has any gas particle.

Snapshot offsets

See merge request !710

When writing the boxsize and time to the snapshot headers, convert the values to the snapshot unit system.

Added canvas RTD documentation of the snapshot content. Added full documentation of the hash-table mechanism.

If the snapshot units and internal units differ, re-scale the cell centres and widths to match the particle data unit system.

Write the centre of each cell, its particle count and global offset to the snapshots. Works without MPI.

When interpolating the 1D tables in the EAGLE cooling model, use a small epsilon for the table edges to avoid out-of-table access.

Improve the check for cooling tables in the EAGLE model to prevent the constant re-loading of tables every step at z=0.

Correct additional eagle cooling abundances

Closes #509

See merge request !708