Implement is_NaN for all rings with NaN elements

[rwst]

Elements of the following rings can't be checked for NaN:

• CC (RR can)
• CDF (RDF can)
• CIF (RIF can)
• Ball fields

Other rings don't have NaN so there is no need for them.

The ticket might add back-conversions from complex NaN to real NaN (which should always be possible).

Component: numerical

Author: Ralf Stephan

Branch/Commit: 920e5ad

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/23001

[rwst]

Description changed:

--- 
+++ 
@@ -6,3 +6,5 @@
 * Ball fields
 
 Other rings don't have `NaN` so there is no need for them.
+
+The ticket might add back-conversions from complex NaN to real NaN (which should always be possible).

[rwst]

Branch: u/rws/implement_is_nan_for_all_rings_with_nan_elements

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

ca839ea	23001: is_NaN for ball fields
5f8b703	23001: NaN back-conversion with interval fields

[sagetrac-git]

Commit: 5f8b703

[rwst]

Description changed:

--- 
+++ 
@@ -2,7 +2,6 @@
 * `CC` (`RR` can)
 * `CDF` (`RDF` can)
 * `CIF` (`RIF` can)
-* `RLF`, `CLF`
 * Ball fields
 
 Other rings don't have `NaN` so there is no need for them.

[rwst]

Author: Ralf Stephan

[tscrim]

comment:5

LGTM.

[tscrim]

Reviewer: Travis Scrimshaw