docs: create basic usage guide

Author: Eoghan O'Connell

docs/sec_basic_use.rst

@@ -0,0 +1,69 @@
+Basic Use of qpretrieve
+=======================
+
+
+
+.. code-block:: python
+
+	import qpretrieve
+
+	# load your experimental data
+	edata = np.load("./data/hologram_cell.npz")  # example data from the qpretrieve repository
+	data = edata["data"]
+	data_bg = edata["bg_data"]
+
+	# create an off-axis hologram object, to process the holography data
+	oah = qpretrieve.OffAxisHologram(data)
+	oah.run_pipeline(filter_name=filter_name, filter_size=filter_size)
+	# process background hologram
+	oah_bg = qpretrieve.OffAxisHologram(data=data_2d_bg)
+	oah_bg.process_like(oah)
+
+	print(f"Original Hologram shape: {data.shape}")
+	print(f"Processed Field shape: {oah.field.shape}")
+
+	# Now you can look at the phase data
+	phase_corrected = oah.phase - oah_bg.phase
+
+
+
+Differences between old (2D) and new (3D) image processing
+----------------------------------------------------------
+
+qpretrieve used to only processed 2D array layout. Since version (0.4.0), it accepts
+3D array layout, and always returns the data as 3D, regardless of the input
+array layout.
+
+New version example code and output:
+....................................
+
+.. code-block:: python
+
+	import qpretrieve  # versions older than 0.4.0
+
+	hologram = np.ones(shape=(256, 256))
+	oah = qpretrieve.OffAxisHologram(hologram)
+	oah.run_pipeline()
+	assert oah.field.shape == (1, 256, 256)  # <- now a 3D array is returned
+	# if you want the original array layout (2d)
+	field_2d = oah.get_array_with_input_layout("field")
+
+	# this means you can input 3D arrays
+	hologram_3d = np.ones(shape=(50, 256, 256))
+	oah = qpretrieve.OffAxisHologram(hologram_3d)
+	oah.run_pipeline()
+	assert oah.field.shape == (50, 256, 256)  # <- always a 3D array
+
+
+Old version example code and output:
+....................................
+
+.. code-block:: python
+
+	import qpretrieve  # versions older than 0.4.0
+
+	hologram = np.ones(shape=(256, 256))
+	oah = qpretrieve.OffAxisHologram(hologram)
+	oah.run_pipeline()
+	assert oah.field.shape == hologram.shape
+

docs/sec_getting_started.rst

@@ -6,4 +6,5 @@ Getting started
   :maxdepth: 2
 
   sec_installation
+  sec_basic_use
   sec_userapi