Unit inference reporting (#130)

* Update FLAMINGO test for index units

* Improve unit reporting

* improve docs on failing unit inference

* add missing tng sim doc page

docs/supported_datasets/tng.md

@@ -0,0 +1,24 @@
+# The TNG Simulation Suite
+
+## Overview
+The IllustrisTNG project is a series of large-scale cosmological
+magnetohydrodynamical simulations of galaxy formation. The data is
+available at [www.tng-project.org](https://www.tng-project.org/).
+
+## Demo data
+
+Many of the examples in this documentation use the TNG50-4 simulation.
+In particular, we make a snapshot and group catalog available to run
+these examples. You can download and extract the snapshot and its group
+catalog from the TNG50-4 test data:
+
+``` bash
+wget https://heibox.uni-heidelberg.de/f/dc65a8c75220477eb62d/?dl=1 -O snapshot.tar.gz
+tar -xvf snapshot.tar.gz
+wget https://heibox.uni-heidelberg.de/f/ff27fb6975fb4dc391ef/?dl=1 -O catalog.tar.gz
+tar -xvf catalog.tar.gz
+```
+
+The snapshot and group catalog should be placed in the same folder.
+Then you can load the snapshot with `ds = load("./snapdir_030")`. The group catalog should automatically be detected,
+otherwise you can also pass the path to the catalog via the `catalog` keyword to `load()`.

docs/tutorial/observations.md

@@ -23,6 +23,11 @@ Choosing a subset means that the data size is small and easy to work with. We de
 
 First, we load the dataset using the convenience function `load()` that will determine the appropriate dataset class for us:
 
+!!! info "Missing units"
+    Below snippet will report missing units for some fields. This is expected.
+    Those fields that cannot have their units determined automatically at this point and carry the unit **unknown**.
+    See [Units](../../units) for more information.
+
 
 ```pycon title="Loading a dataset"
 >>> from scida import load

docs/units.md

@@ -16,10 +16,33 @@ ds = load("TNG50-4_snapshot")
 ```
 
 will automatically attach units to the data. This can be deactivated by passing "units=False" to the load function.
-By default, code units are used, alternatively, cgs conversions can be applied by passing "units='cgs'".
+By default, code units are used, alternatively, cgs conversions can be applied by passing "units='cgs'" (experimental).
 
 Units are introduced via the [pint](https://pint.readthedocs.io/en/stable/) package, see there for more details.
 
+Sometimes, the units cannot be inferred or parsed.
+This will be indicated in the output following the call to `load()`, e.g. as:
+
+``` pycon
+Missing units for 1 fields.
+Fields with missing units:
+  - /PartType0/FieldWithoutUnits (missing)
+```
+
+You can obtain more information on the cause by setting
+
+``` pycon
+>>> import logging
+>>> logging.getLogger().setLevel(logging.DEBUG)
+```
+
+before calling `load()`.
+
+Units for custom datasets can also be manually be specified using unit files, see [here](configuration.md#unit-files).
+
+
+
+## Using data with units
 
 ``` pycon
 >>> gas = ds.data["PartType0"]