Fixed seek bug in CtbRawFile (#373)

- seek to the edge of a file + read_into would select the wrong file
- Visible for example in f.read_frame(index) in python

Added fix, release notes and a small test. 

closes #371

Co-authored-by: AliceMazzoleni99 <alice.mazzoleni@psi.ch>

docs/src/python/file/pyCtbRawFile.rst

@@ -6,6 +6,10 @@ Read analog, digital and transceiver samples from a raw file containing
 data from the Chip Test Board. Uses :mod:`aare.transform` to decode the 
 data into a format that the user can work with. 
 
+``read_frame(index)`` reads the frame at the zero-based index across the raw
+subfiles and advances the position to ``index + 1``. A subsequent
+``read_frame()`` reads the following frame, including across subfile boundaries.
+
 .. code:: python
 
     import aare
@@ -22,4 +26,4 @@ data into a format that the user can work with.
     :members:
     :undoc-members:
     :show-inheritance:
-    :inherited-members:
+    :inherited-members: