Change null_policy to handle small non-zero values

[dcslagel]

Description:

This pull-request fixes issue: 'Incorrect parsing of null values with negative values' #427

• This change filters non-zero values after '-0.0..' for null_policy='aggressive'. Example: -0.0733
• Test, test_null_policy_small_non_zero_neg_nums is added.

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                 407     59    86%
lasio/las_items.py           199     31    84%
lasio/las_version.py          50     14    72%
lasio/reader.py              396     25    94%
lasio/writer.py              175     10    94%
----------------------------------------------
TOTAL                       1407    205    85%

--

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]

Thanks @dcslagel