Massive PR, datamodel change.

[brisvag]

This PR started as a rework of some classification function, and soon it was clear that many things in the datamodel and project structure needed fixing. I changed a lot of stuff, but here's a (probably incomplete) writeup:

• changed to a data strcture with DataSet at its core. DataBlocks are now directly contained in it, and have themselves a reference to what "volume" (previously DataCrate object) they belong to. This greatly simplify looping and such, since we're not working with an actual nested structure.
• naming of blocks and volumes is now used as unique identifiers, which will make it easy for merging volumes and avoid datablock conflicts
• previous DispatchList functionality is still present, but only when needed (not directly on the dataset itself, but for example when calling DataSet.particles)
• depictors model changed with the rest: now only datablocks have depictors; they can have multiple depictors, and multiple modes (for example, for plots)
• minimal mesh depiction
• classification now does ome vectorised fancy stuff. Some of the representation is slighly broken as is due to the big changes to depictor structure, but it works
• removed a lot of legacy code that was useless (all the merge/stack stuff, for example)

Tests work but things may have broken where tests don't cover... will need some more time to figure it out!

[codecov]

Codecov Report

Merging #68 (271f2c3) into develop (bb34142) will increase coverage by 1.54%.
The diff coverage is 68.83%.

@@             Coverage Diff             @@
##           develop      #68      +/-   ##
===========================================
+ Coverage    68.51%   70.06%   +1.54%     
===========================================
  Files           85      101      +16     
  Lines         1639     1974     +335     
===========================================
+ Hits          1123     1383     +260     
- Misses         516      591      +75     

Impacted Files	Coverage Δ
peepingtom/__main__.py	0.00% <ø> (ø)
peepingtom/gui/__init__.py	0.00% <ø> (ø)
peepingtom/gui/gui.py	0.00% <ø> (ø)
peepingtom/gui/particle_selector.py	0.00% <0.00%> (ø)
peepingtom/io_/utils/generic.py	100.00% <ø> (+35.00%)	⬆️
peepingtom/io_/utils/star_conventions.py	100.00% <ø> (ø)
peepingtom/io_/write/star/star.py	95.83% <ø> (ø)
peepingtom/io_/read/read.py	18.29% <27.77%> (-1.19%)	⬇️
peepingtom/depictors/viewer.py	28.12% <28.12%> (ø)
peepingtom/depictors/pyqtgraph/plotdepictor.py	35.00% <35.00%> (ø)
... and 119 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f54cd03...271f2c3. Read the comment docs.

[alisterburt]

Looks good overall - seems like working with datasets will be easier with this architecture! Few tiny questions but nothing blocking, happy for you to merge if you're happy!