Expanding compatibility for seabed slope and current loads:

Through various changes, mainly in Line and Catenary, seabed
slope and current load capabilities should now work in general
including with taut line sections under currents.

Line
- Fixed some disconnects and bugs throughout related to seabed
  contact, and to ensure all tests still pass after new
  capabilities.
- Adding some hasty steps to ensure the seabed is ignored for
  lines that are off the seabed and experiencing current loads
  (especially a factor for taut lines, which may have huge
  rotations of the w vector).
- Simplified current force calculation to apply drag equation
  to line segments rather than nodes.

Catenary
- Add support for sloped seabed profile to work in more
  cases, such as when end B is below end A.
- Added support for sloped seabed in slack (L shaped) profile.
- Improved handling of different profile types for different
  scenarios and removed some unintended behavior when dealing
  with end B lower than end A and incorrect input of height
  off seabed.
- Internally replaced use of -CB for height off seabed with
  dedicated hA and hB values for each end (less confusing).
- More streamlining and some misc cleanup throughout code.
- dsolve2 now truncates empty parts of solve record list.
- New test added for sloped seabed in test_catenary.py.