Support for Python byte type

[kshitij-v-mehta]

We have a use case with GTC in which we need to store images in a BP file. It would be nice to add a feature that supports Python variables of class bytes (not the same as bytearrays).

As Python bytes are not natively supported, the current approach requires converting an image represented as a bytes array to a numpy uint8 type and writing it to file. Conversely, to read it back, we need to convert the uint8 array to bytes.

The following pseudocode shows a snippet on how this is currently done.

# Perform matplotlib plotting

# Get a bytes object
b = io.BytesIO()

# Write plot to the bytes buffer and return it
plt.savefig(b, format='png')

# Higher level function receives b and converts it into a numpy array.
# Write the numpy array as a uint8 array
dt = np.dtype(np.uint8)
N = len(b)
data = np.frombuffer(b, dtype=dt, count=N, offset=0)
adios_fh.write(varname, data, [N], [0], [N])