Refac test implementation (#146)

With this PR, I propose a major refactoring of the test implementation,
mostly to drop `QEDcore.jl` as a test dependency. See #144 for details.

## TODO (unit tests itself)

- [x] particle: check only particle properties
- [x] particle: implement and check `propagator` 
- [x] particle: implement and check `base_state`
- [x] process: moving parts that do not belong to processes (marked in
the code by `TODO`)
- [x] particle stateful: implement some tests
- [x] cross section: remove coordinate-based evaluation (is checked in
PSP tests)
- [x] core_compat: move to `QEDcore.jl`

## TODO (test implementation)

- [x] psp: make `TestPhaseSpacePoint(...,moms)` type-stable
- [x] psp: implement proper in- and out-psps
- [x] particle: implement massless particles
- [x] psl: implement failing PSL

## TODO (general)

- [x] consider testing everything for `(TestMomentum{T},
TestMomentumMutable{T})` where `T<:Union{Float16,Float32, Float64}` (not
just for `TestMomentum{Float64}`)


## Final remarks

Solves #144. 

Makes #140 easier to achive.

---------

Co-authored-by: Uwe Hernandez Acosta <[email protected]>
Co-authored-by: Anton Reinhard <[email protected]>

Project.toml

@@ -1,6 +1,12 @@
 name = "QEDbase"
 uuid = "10e22c08-3ccb-4172-bfcf-7d7aa3d04d93"
-authors = ["Uwe Hernandez Acosta <[email protected]>", "Simeon Ehrig", "Klaus Steiniger", "Tom Jungnickel", "Anton Reinhard"]
+authors = [
+    "Uwe Hernandez Acosta <[email protected]>",
+    "Simeon Ehrig",
+    "Klaus Steiniger",
+    "Tom Jungnickel",
+    "Anton Reinhard",
+]
 version = "0.3.0"
 
 [deps]
@@ -19,18 +25,16 @@ ArgCheck = "2.3.0"
 ConstructionBase = "1"
 DocStringExtensions = "0.8.5, 0.9"
 PhysicalConstants = "0.2.1"
-QEDcore = "0.2"
 SimpleTraits = "0.9.4"
 StaticArrays = "1.2.13"
 julia = "1.10"
 
 [extras]
 Pkg = "44cfe95a-1eb2-52ea-b672-e2afdf69b78f"
-QEDcore = "35dc0263-cb5f-4c33-a114-1d7f54ab753e"
 SafeTestsets = "1bc83da4-3b8d-516f-aca4-4fe02f6d838f"
 Suppressor = "fd094767-a336-5f1f-9728-57cf17d0bbfb"
 Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 oneAPI = "8f75cd03-7ff8-4ecb-9b8f-daf728133b1b"
 
 [targets]
-test = ["SafeTestsets", "Test", "Pkg", "Suppressor", "QEDcore"]
+test = ["SafeTestsets", "Test", "Pkg", "Suppressor"]