Nptdms

Fixes numpy bool8 deprecation warning.

* Support writing tdms_index files and directly reading tdms_index files
* Add TdmsWriter.defragment method for rewriting a TDMS file
* Drop support for Python 2.7 and 3.6

What's Changed
* Support all remaining strain gauge configurations (https://github.com/adamreeve/npTDMS/pull/268)
* Add option to specify the TDMS format version to write (https://github.com/adamreeve/npTDMS/pull/265)
* Fix reading files with truncated metadata in the final segment (https://github.com/adamreeve/npTDMS/pull/273)
* Support IPython auto-completion for group and channel names (https://github.com/adamreeve/npTDMS/pull/259)

**Full Changelog**: https://github.com/adamreeve/npTDMS/compare/1.4.0...1.5.0

* Improve performance reading data into numpy arrays
* Fix getting channel time track when using raw timestamps
* Fix numpy deprecation warnings

Bug fix to handle when the first segment does not have kTocNewObjList set

* Handle segments containing data for a single string channel with the interleaved flag set
* Improve handling of incomplete segment data