Add keyword arguments to writer.py:write to accommodate #412

[kinverarity1]

This PR adds some keyword arguments to the writer.py:write function to accommodate the request in issue #412.

For example:

import lasio
import lasio.examples

las = lasio.examples.open("sample.las")
from sys import stdout

las.write(stdout, lhs_spacer="", len_numeric_field=-1, spacer=" ")

It adds the option len_numeric_field=-1 which is needed to constrain the column width. spacer=" " sets the width between numbers. lhs_spacer="" makes the width of the whitespace on the left hand side to zero characters.

~ASCII -----------------------------------------------------
1670.00000 123.45000 2550.00000 0.45000 123.45000 123.45000 110.20000 105.60000
1669.87500 123.45000 2550.00000 0.45000 123.45000 123.45000 110.20000 105.60000
1669.75000 123.45000 2550.00000 0.45000 123.45000 123.45000 110.20000 105.60000