feat: Add methods for writing variable data

docs/source/design-howto.rst

@@ -15,7 +15,8 @@ overhead for you, but it enables you to choose between:
 
 Depending on your use-case, using **memory-mapped I/O** can be **very
 efficient**. It also allows for **limited editing of EnSight files**
-(eg. transforming coordinates or variable values in-place).
+(eg. transforming coordinates or variable values in-place). Regular
+I/O can be used for **appending new parts and variable data**.
 
 The following code example demonstrates this feature:
 
@@ -27,11 +28,16 @@ The following code example demonstrates this feature:
     geofile = case.get_geometry_model()
     part_ids = geofile.get_part_ids()
     part = geofile.get_part_by_id(part_ids[0])
+    variable = case.get_variable("RTData")
 
-    # (1) traditional I/O
+    # (1.1) read-only traditional I/O
     with geofile.open() as fp_geo:
         nodes = part.read_nodes(fp_geo)  # owned buffer
 
+    # (1.2) read-write traditional I/O
+    with variable.open_writable() as fp_var:
+        variable.ensure_data_for_part(fp_var, part.part_id, 0.0)  # append array with default value if needed
+
     # (2.1) read-only memory-mapped I/O
     with geofile.mmap() as mm_geo:
         nodes = part.read_nodes(mm_geo)  # buffer backed by read-only mmap
@@ -228,7 +234,7 @@ Please see ``tests/test_write_geometry.py``, essentially you will need to do:
 Copying data from other case
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-There is a convenience method to append parts (geometry only) from different case:
+There are convenience methods to append part geometry and variable data from a different case:
 
 ::
 
@@ -242,8 +248,8 @@ There is a convenience method to append parts (geometry only) from different cas
 
     >>> source_part = source_geo.get_part_by_name("my_part")
     >>> dest_case.append_part_geometry(source_case, [source_part])
+    >>> dest_case.copy_part_variables(source_case, [source_part], ["velocity", "pressure"])
 
-    >>> source_geo = source_case.get_geometry_model()  # read updated geometry file
 
 Code examples
 -------------
@@ -253,6 +259,9 @@ other data formats. These are mostly useful for reference as they
 output data in text format, making them unsuitable for production use
 with large models.
 
+.. tip::
+    See the ``tests/`` directory in the repository for more code examples.
+
 .. automodule:: ensight2obj
     :members: