Clean up and fix a nasty bug in tcpip instr session.

[MatthieuDartiailh]

The term char was incorrectly converted, which lead to read incomplete messages. Also optimize some routines to use bytearray and some general pep 8 formatting, and prevent a hard fail if one instrument connected in USB is not properly configured (can happen on windows with the wrong driver installed)

@skrchnavy if you have time to review it would be nice. This fixes the issue I have seen in #120 .

[MatthieuDartiailh]

Merging this is really an improvement.

bors r+

[bors[bot]]

Build succeeded

• continuous-integration/travis-ci/push