On write, update step vars if curve idx changed

[dcslagel]

Description:

This should complete Remove side-effects from write() #268.

This is a first iteration at updating the STEP variable as part of the write() process but only in the following cases:

1. an initial curve index was not read from a las file (las.index_initial)
2. or the curve index has changed during processing
3. or if the STOP value doesn't match the final index value

I am not sure if case #3 should trigger updating the step variables. It looks like many of the test case LAS files have STOP values that don't match the final index value or the final index value - 1 STEP. Let me know if these needs to be modified.

Test Results:

Python 3.9

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                 431     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              166     10    94%
----------------------------------------------
TOTAL                       1422    204    86%

--

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

Thank you,
DC

[kinverarity1]

1. • STEP should definitely be auto-calculated if LASFile is created from scratch.

2. I'll look at the failing tests tonight but think that we can safely change them as long as the expected output makes sense...

[kinverarity1]

Looks great, thanks!