Usage of matplotlib

[janscience]

The algorithms in the thunderfish modules should be usable without automatic import of matplotlib.pyplot - maybe matplotlib is not available (on a server) but you nevertheless want to use the algorithms. Therefore:

• Each module may define functions that do plot some stuff.
• If a module function implicitly uses matplotlib.pyplot
  by operating on an axis, the module does NOT need to import matplotlib.
• If a function uses matplotlib.pyplot directly, then you need to import matplotlib.pyplot in the module -
  but then catch the ImportError as described below.
• Importing matplotlib.pyplot in the main() function for demonstration is no problem.
• Importing matplotlib.mlab is allowed if you need the mlab functions for basic (non-plotting) functionality of your script.
• Catch the ImportException if you really need to import some matplotlib stuff in the module, e.g.

try:
    import matplotlib.pyplot as plt
except ImportError:
    pass