add test for new field class

obviously, it's basically empty. all I need to do now is generate the
field class, then generate a C++ main file that uses it, then compare
the results against pyFFTW

tests/test_field_class.py

+import numpy as np
+import matplotlib.pyplot as plt
+import pyfftw
+import bfps
+import bfps.tools
+
+def main():
+    kdata = pyfftw.n_byte_align_empty(
+            (32, 32, 17),
+            pyfftw.simd_alignment,
+            dtype = np.complex64)
+    rdata = pyfftw.n_byte_align_empty(
+            (32, 32, 32),
+            pyfftw.simd_alignment,
+            dtype = np.float32)
+    c2r = pyfftw.FFTW(
+            kdata.transpose((1, 0, 2)),
+            rdata,
+            axes = (0, 1, 2),
+            direction = 'FFTW_BACKWARD',
+            threads = 8)
+    kdata[:] = bfps.tools.generate_data_3D(32, 32, 32, dtype = np.complex64)
+    c2r.execute()
+    fig = plt.figure()
+    a = fig.add_subplot(111)
+    a.set_axis_off()
+    a.imshow(rdata[:, 4, :], interpolation = 'none')
+    fig.tight_layout()
+    fig.savefig('tst.pdf')
+    return None
+
+if __name__ == '__main__':
+    main()
+