Add test for writing file with empty ~P section (#67)

[kinverarity1]

No description provided.

[kinverarity1]

The original error from #67

Traceback (most recent call last):
 File "memgauge2las.py", line 46, in 
 l.write(f,2)
 File "/home/bdi/projects/lasfbe_dev/lasio/lasio/las.py", line 353, in write
 section_widths = get_section_widths("params", self.params, version)
 File "/home/bdi/projects/lasfbe_dev/lasio/lasio/las.py", line 790, in get_section_widths
 section_widths["left_width"] = max([len(i.mnemonic) for i in items])
 ValueError: max() arg is an empty sequence

[kinverarity1]

The error I'm getting in the test suite:

================================== FAILURES ===================================
_______________________ test_write_sample_empty_params ________________________

    def test_write_sample_empty_params():
        l = read(egfn("sample_write_empty_params.las"))
>       l.write(StringIO(), version=2)

lasio\test_write.py:166:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
lasio\las.py:353: in write
    section_widths = get_section_widths("params", self.params, version)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

section_name = 'params', section = {}, version = 2, middle_padding = 5

    def get_section_widths(section_name, section, version, middle_padding=5):
        '''Find minimum section widths fitting the content in *section*.

        Args:
          section_name (str): either "version", "well", "curves", or "params"
          section (dict|list): section items
          version (float): either 1.2 or 2.0

        '''
        section_widths = {}
        if isinstance(section, dict):
            items = section.values()
        elif isinstance(section, list):
            items = list(section)

>       section_widths["left_width"] = max([len(i.mnemonic) for i in items])
E       ValueError: max() arg is an empty sequence

lasio\las.py:790: ValueError
===================== 1 failed, 23 passed in 3.67 seconds =====================