move write_coefficients to interpolator
add c function generator for periodic grids
add c function generator for generic grids

also, some niceties in the way this is used in the final C file that the
interpolator writes.
add interpolation sum

supposedly this should work. now I just have to test it.
add methods to compile shared library for interpolator
fix cinterpolate so there's no segfault
remove all segfaults

resulting plot is smooth, and there are no segfaults for arbitrary
dimensions of the input array.
finish up clib

some minor aesthetical thing, and testing with the channel database
works.
TODO:
put a test in examples, and test the derivatives too.
add wrapper for getRaw type calls
add test for interpolation

and for rawData, although I'm not sure that one makes any sense...
minor fix: title for interp test axes
Merge branch 'splines-ctypes-acceleration' into develop
don't perform modulo operation in interpolation

people should provide field_offsets instead
add temporary dbinfo generator
interpolate: better handling of indices

at the moment, function will only work properly as long as the points
are within the domain of the field, when shifted properly with the
offset. i.e. I'm ignoring the case when field is periodic and there are
points outside the initial domain.
This should be fixed in the future.
remove debug message
play around with the interp test
add 1D test
add test to measure divergence of interpolated velocity
Merge branch 'check_interp' into develop