Add get_include_path

While we could make users use importlib.resources.files manually,
it is annoying, so provide a helper routine (this is based on
the logic we use in Sherpa).

CHANGELOG.md

@@ -12,6 +12,11 @@ from the refactoring done in 0.0.30, and move the include directory
 into xspec_models_cxc/ so it is more-obvious why is is installed
 (although it probably should be done more as some form of a data-file).
 
+The module has gained the get_include_path() routine to provide access
+to the include file, which will hopefully be needed to support access
+to XSPEC user models (I have got this working for Sherpa so we should
+be able to do something similar here).
+
 ## 0.0.30
 
 Potential build improvements, including support for using the CXC

template/__init__.py

@@ -8,14 +8,15 @@
 The XSPEC model library is automatically initialized when the first
 call is made, and not when the module is loaded.
 
-There are three types of symbols in this package:
+There are four types of symbols in this package:
 
 1. model functions, such as `apec` and `TBabs`, and `tableModel` for
    table models.
 2. routines that set or get values such as the abundance
    table (`abundance`), cross-section table (`cross_sections`),
    cosmology (`cosmology`), and the chatter level (`chatter`).
 3. routines about the models: `info` and `list_models`.
+4. routines about the API: `get_include_path`.
 
 Examples
 --------
@@ -131,7 +132,9 @@
 
 from dataclasses import dataclass
 from enum import Enum, auto
+from importlib import resources
 import logging
+from pathlib import Path
 from typing import List, Optional, Sequence
 
 try:
@@ -324,3 +327,10 @@ def list_models(modeltype: Optional[ModelType] = None,
         out.add(k)
 
     return sorted(out)
+
+
+def get_include_path() -> Path:
+    """Return the location of the C++ include file."""
+
+    path = resources.files("xspec_models_cxc.include")
+    return path / "xspec_models_cxc.hh"