Reject raw files without padding if discard partial is not set (#374)

We have no code in aare to deal with RawFiles that have missing packets
if they are not padded to full size or frames with packet loss dropped.

Added: 
- check that we don't try to read files that are not supported
- python bindings for frame discard policy

at least we get the check in: 
closes #171

---------

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

docs/src/python/file/pyRawFile.rst

@@ -3,6 +3,13 @@ RawFile
 
 .. py:currentmodule:: aare
 
+Reading requires frame padding to be enabled 
+or ``Frame Discard Policy`` to be ``discardpartial``.  Construction raises
+``RuntimeError`` with the master path before opening data subfiles if this
+requirement is not met. This applies to JSON and legacy ``.raw`` masters,
+including when opened through ``File``. ``RawMasterFile`` can still be used
+to inspect their metadata.
+
 ``total_frames`` and ``len(reader)`` report the minimum actual frame count
 across all selected subfiles and ROIs, including each subfile's complete
 ``.raw`` series. ``read()`` uses this count, and ``read_n()`` and