Encodes to and decodes from base128.

## Project description

If called from command line, this encodes a binary file into base128.
If imported from python it provides a base128 class to do the same.

An instance of base128 can be used to convert to and from base128 encoding.

Encoding: The python package bitarray is used to insert a 0 bit every 8
bits of the data. Bitarray cares to shift the bits to make room for the
new bit. This is done in chunks.
The length in bits mod 8 can become greater than zero for chunks of size
not equal to a multiple of 7. So chunksize must be a multiple of 7.
Even if chunksize is a multiple of 7 the last chunk
likely has to be padded to reach a multiple of 8 after encoding.
The amount of padding can be expressed as a function of the original data
length mod chunksize (modchunk). modchunk is added as an
additional byte at the end of the encoding. To make this byte also
base128, we require chunksize<=128.

If chars is provided, the resulting 7-bit numbers are
used as indices to map to entries of chars.
With bytes chars the resulting chunks will be integer lists
and possibly still need to be typed to bytes for further processing::

with open('tstenc.txt','wb') as f: f.write(b'\n'.join([bytes(x) for x in encoded]))