Move strt/stop/step config to its own function

[dcslagel]

Description:

This is part of Remove side-effects from write() #268 but not the whole task.

Move strt/stop/step config/change from writer.py::write() to its own function as part of the LASFile class: update_start_stop_step()

For this checkin, las.update_start_stop_step() is included in writer.py::write() so the overall behavior is the same.

Test Results:

Name                       Stmts   Miss  Cover
----------------------------------------------
lasio/__init__.py             13      2    85%
lasio/convert_version.py      20     20     0%
lasio/defaults.py             11      0   100%
lasio/examples.py             42     10    76%
lasio/excel.py                88     34    61%
lasio/exceptions.py            6      0   100%
lasio/las.py                 428     60    86%
lasio/las_items.py           199     29    85%
lasio/las_version.py          50     14    72%
lasio/reader.py              396     25    94%
lasio/writer.py              159      9    94%
----------------------------------------------
TOTAL                       1412    203    86%
Coverage XML written to file coverage.xml

--

Let me know if this change could be accepted (or rejected) or
needs some additional changes to be approved and merged.

Thank you,
DC