documentation & other code improvements

While working on a documentation setup with sphinx and rtd-theme WIP, I noticed that many parts of the pmb code-base have no docstrings.

running flake8 with the flake8-docstrings extension installed gave the folling report:

108   D100 Missing docstring in public module
2     D101 Missing docstring in public class
5     D102 Missing docstring in public method
371   D103 Missing docstring in public function
15    D104 Missing docstring in public package
1     D107 Missing docstring in __init__
41    D200 One-line docstring should fit on one line with quotes
34    D202 No blank lines allowed after function docstring
2     D204 1 blank line required after class docstring
215   D205 1 blank line required between summary line and description
62    D208 Docstring is over-indented
55    D209 Multi-line docstring closing quotes should be on a separate line
102   D210 No whitespaces allowed surrounding docstring text
3     D301 Use r""" if any backslashes in a docstring
234   D400 First line should end with a period
26    D401 First line should be in imperative mood; try rephrasing
2     D403 First word of the first line should be properly capitalized
1     E127 continuation line over-indented for visual indent
7     E128 continuation line under-indented for visual indent
1     E231 missing whitespace after ','
6     E261 at least two spaces before inline comment
2     E302 expected 2 blank lines, found 1
76    E501 line too long (83 > 79 characters)
2     E502 the backslash is redundant between brackets
2     E722 do not use bare 'except'
55    F401 '.config' imported but unused
3     W293 blank line contains whitespace

Full report with references to files needing improvement: pmb_master_flake8.txt