Some updates to rounding behavior when writing for issue 453

[dcslagel]

Description:

This pull-request resolves the issues identified in rounding issues when writing las file #453.

1. Updates for rounding behavior when writing:

• If we are getting STRT and STOP from the data then format them to a
  standard precision. Current default precision is "%.5f".
• If STRT and STOP are passed in with values, don't format them because
  we assume they are at the user's expected precision.
• Update failing test cases to have the correct STOP value.
• Add a test case for the case where a LAS file does not have a correct
  STOP value.
• Move updating units to a separate LASFile method so that units are
  always synced between las.curves.unit and las.well..unit.

2. Enable writing empty LAS file:

• Handle exceptions on empty data
• Add test case for writing an empty LAS file: test_write_empty_las()

Testing 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                 436     59    86%
lasio/las_items.py           199     29    85%
lasio/las_version.py          50     14    72%
lasio/reader.py              396     25    94%
lasio/writer.py              171      9    95%
----------------------------------------------
TOTAL                       1432    202    86%


- benchmark: 1 tests -------------------------------
Name (time in s)                Min     Max    Mean  
----------------------------------------------------
test_read_v12_sample_big     944.9445  957.4294  950.0452
---------------------------------------------------

--
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]

Looks great thank you.