bugfix: activate_time naw seems to work as expected
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
add __enter__ and __exit__ methods to functions with an unload method
fix seed numpy type interaction with cython
tidy some reader syntax
fix container class warnings

UNTESTED!
fix seed numpy type interaction with cython
tidy some reader syntax