Allow different data types per curve in data section reader

[kinverarity1]

This commit changes the data section reader function (reader.py:read_data_section_iterative) to return a generator which yields the data ndarray (1D) for each curve in turn. The old behaviour was for the function to return a 2D ndarray.

It also:

• reshaping of the data array to 2D now occurs solely inside the data section reader function, which has a new argument "n_columns" to allow this to happen
• adds dtypes kwarg to las.py:LASFile.read
• add reader.py:identify_dtypes_from_data function to implement dtypes="auto" (default value) by using the first row of
  data to automatically identify column data types
• changes to las.py:LASFile.read to cater for the above

Consequences:

• This PR will fundamentally resolve Data section containing non-numeric chars is parsed entirely as str #439 Data section containing non-numeric chars is parsed entirely as str
• Provides a basis for fixing e.g. Time-indexed LAS file #416 Time-indexed LAS file
• Possibly partially resolve (and provide a way to fix) NULL value replacing valid value in DEPT #227 NULL value replacing valid value in DEPT
• @dcslagel Eventually we will need to merge this into PR Add a numpy engine for reading using numpy.genfromtxt() #452 (numpy engine) or rebase - I'm happy to help out with this.

Details:

• Coverage increased from 85% to 86%
• Benchmark test from 1.4 s to 1.0 s ?

---------- coverage: platform linux, python 3.6.13-final-0 -----------
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                 419     60    86%
lasio/las_items.py           190     29    85%
lasio/las_version.py          50     15    70%
lasio/reader.py              435     27    94%
lasio/writer.py              171      9    95%
----------------------------------------------
TOTAL                       1445    206    86%
Coverage XML written to file coverage.xml


----------------------------------------------- benchmark: 1 tests ----------------------------------------------
Name (time in s)                Min     Max    Mean  StdDev  Median     IQR  Outliers     OPS  Rounds  Iterations
-----------------------------------------------------------------------------------------------------------------
test_read_v12_sample_big     1.0652  1.0758  1.0700  0.0041  1.0694  0.0062       2;0  0.9346       5           1
-----------------------------------------------------------------------------------------------------------------

[dcslagel]

This change looks good. I had one small change request.