add cython cdef usage comment
limited streamline code implemented:

fused dispatch to _py_streamline does not work at runtime!
code works, streamlines appear wrong though
fixed dipole test, field lines look good, but 2d field doesn't seem to work like I hoped
fix obound memoryviews
tidy up test program
put plot_field_lines into viscid.plot.mpl
add calculator.seed module for streamline seeds
stop drawing streamlines if magnitude of vector is 0
break apart monolithic python module
fix streamline test
add bounds checking back into all python funcs
Merge branch 'feature/field_lines' into develop