Improve versatility and robustness of `cpptype`, and document it in the user guide

This MR improves the implementation and interface of cpptype, adds a number of tests, extends the STL API reflections using it, and introduces a user guide section on API reflection.

lang.cpptype

• Fix correct cloning of CppType, as required for (de)constify, by keeping and passing on the raw template parameters
• Customize the template formatter to reject superfluous template arguments
• Have cpptype return a CppTypeFactory object which is used to instantiate class templates, and apply const and ref qualifications
• Introduce the convenience subblass CppClass of AugExpr to simplify the definition of API reflections for C++ classes

lang.cpp.std

• Update reflections of C++ STL classes to use the new features of cpptype

Composer

• Remove the undocumented cpptype method from the composer, to avoid confusion about its name

Documentation

• Add a user guide section on cpptype and API reflection using CppClass
• Add myst-nb as the MyST markdown engine in order to parse and execute Jupyter and text-based notebooks

src/pystencilssfg/lang/types.py

 from __future__ import annotations
-from typing import Any, Iterable, Sequence, Mapping
+from typing import Any, Iterable, Sequence, Mapping, Callable
 from abc import ABC
 from dataclasses import dataclass
 from itertools import chain
@@ -107,7 +107,7 @@ class CppType(PsCustomType, ABC):
 
 def cpptype(
     typestr: str, include: str | HeaderFile | Iterable[str | HeaderFile] = ()
-) -> type[CppType]:
+) -> Callable[..., CppType | Ref]:
     headers: list[str | HeaderFile]
 
     if isinstance(include, (str, HeaderFile)):
@@ -121,15 +121,14 @@ def cpptype(
         template_string = typestr
         class_includes = frozenset(HeaderFile.parse(h) for h in headers)
 
-    class TypeClassFactory(TypeClass):
-        def __new__(cls, *args, ref: bool = False, **kwargs):
-            obj = TypeClass(*args, **kwargs)
-            if ref:
-                return Ref(obj)
-            else:
-                return obj
+    def factory(*args, ref: bool = False, **kwargs):
+        obj = TypeClass(*args, **kwargs)
+        if ref:
+            return Ref(obj)
+        else:
+            return obj
 
-    return TypeClassFactory
+    return staticmethod(factory)
 
 
 class Ref(PsType):