DOC: Clarify UMFPACK warning; use if statements instead of assert
FIX: Shortcut output or catch trivial cases in random_walker

Also:
* New tests to cover these new checks
* Improvements to docstrings and user warnings
* Generalize handling of `sampling` in accordance with docstring
* Some extra whitespace to improve readability
TST: Add unit test for length-2 spacing with rank-2 input
STY: More elegant and maintainable style per code review
Remove check for non-numeric values in spacing.
Merge pull request #984 from JDWarner/randwalk_trivialcheck

Random Walker improvements and checks for trivial cases