Added get_cw_as_str()

New section added to compression which fetches the CW as a binary
string.
The BitString module is slow at creating new BitArrays so it is hoped
that using strings when BitArrays aren't needed will speed this up.
Incorporated get_cw_as_string into MPRa_to_bitstream

The get_cw_as_str function is now incoroporated into the
MPRa_to_bitstream function, in place of the previous get_cw function.
Thus far the speed gained in working with strings initially is lost with
the bitstream.append call.
Began move from BitString to BitArray` module

The BitString module was responsible for making the encoding incredibly
slow, so this has been converted to the BitArray module instead.
Currently BitString is still used when decoding, but this may be changed
to reduce the dependencies in the future.
Begun optimization of get_prediction

The get_prediction function would benefit from optimization, but first
needs a way to be testing in isolation. This may require some
refactoring due to the significant number of parameters currently being
passed, which seems to be not ideal.
Finished converting encoder to use bitarray lib

The encoder module is now entirely using the bitarray lib instead of the
bitstring lib. The decoder is still using bitstring, but will be
converted next in the hope of achieving the same speed increase.
Encode/Decode tests are still valid providing the conversion from
bitarray to BitStream is done between operations.
Updated local sum calculation; non-working

The local sum calculation has been modified to now only be called once
and then generate an array of local sums that can be referenced by other
functions (instead of recalculating every time). The unit tests
involving compression & decompression now fail.
Now ignoring HSI data files