Volume: initial mesh to volume modifier implementation

Using `openvdb::tools::meshToLevelSet` is probably not correct.
I will look into `meshToVolume` next.

Volume: add boilerplate code for Mesh To Volume modifier

Cleanup: remove dead code in point cache and openvdb wrapper

Reviewers: brecht

Differential Revision: https://developer.blender.org/D8988

Cleanup: remove smoke code from pointcache

This code is not used by the new fluid simulation system.
Therefore, it does not make sense to keep it around.
We do want to integrate the fluid system with the generic cache
system eventually, but refactoring the system is easier when there
is less dead code that has to be refactored.

Note, I could not remove BKE_ptcache_id_from_smoke entirely yet,
because the depsgraph seems to expect that object that
uses DEG_add_collision_relations also has a pointcache.
That is because it wants to reset the point cache when any of the
other objects changed (this does not work with fluids currently).

Reviewers: sebbas

Differential Revision: https://developer.blender.org/D8987

Cleanup: remove last uses of WITH_SMOKE

Fix T80943: return early when field to visualize is NULL

Cleanup: reduce variable scopes

Fix T59272: dead particles not included in render, but visible in viewport

The issue was that the pointcache was not storing dead particles,
even though they are displayed. This lead to the rendering issue,
because only alive particles can be read from the point cache in
the frame that is rendered.

This also fixes an issue unrelated to rendering: when dead particles
are displayed, their position is incorrect when some frames are
skipped during playback.

Reviewers: brecht

Differential Revision: https://developer.blender.org/D8907

Cleanup: reduce variable scope

Fix: add versioning to fix incorrectly written customdata

Reviewers: campbellbarton

Differential Revision: https://developer.blender.org/D8903