Immutability for scalar fields

[mjungmath]

This ticket adds the immutability and hashability for scalar fields.

See #30261.

Depends on #30181

CC: @egourgoulhon @tscrim @mkoeppe

Component: manifolds

Keywords: immutability

Author: Michael Jung

Branch/Commit: 1912193

Reviewer: Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/30266

[mjungmath]

Branch: u/gh-mjungmath/immutability_for_scalar_fields

[mjungmath]

Description changed:

--- 
+++ 
@@ -1 +1,3 @@
+This ticket adds the immutability and hashability for scalar fields.
 
+See #30261.

[mjungmath]

Changed keywords from none to immutability

[mjungmath]

Author: Michael Jung

[mjungmath]

New commits:

286d3c6	FreeModuleTensor, Vector: Subclass a new class ModuleElementWithMutability
9df3d22	sage.tensor.modules: Make all zero() and one() elements immutable
4373ea2	FreeModuleTensor: Replace special casing of the immutable zero by is_immutable
a2ee8be	ModuleElementWithMutability.is_immutable, is_mutable: Change def to cpdef
aebaded	Trac #30266: make scalarfields immutable

[mjungmath]

Commit: aebaded

[mjungmath]

Dependencies: #30181

[mjungmath]

comment:4

Is the hash input okay? Or should it be something more unique?

[mkoeppe]

comment:5

Given that ScalarField_repr_ does not print any contents, this hash does not seem to be useful to distinguish non-equal elements. This will degrade the performance of sets to that of lists.

More importantly, it must be ensured that f == g implies hash(f) == hash(g). Because equality does not take names into account but repr does, this is currently not satisfied.