WIP: forward_op.c as a program (move from testsuite to src)
new complex CTF and some minor changes

- rename old ctf() to ctf_acr() as it represents the ctf for constant
amplitude contrast ratio
- do similar with some low-level functions
- try double instead of float for some close-to-zero operations
- new ctf() and related low-level functions to compute the complex
CTF without ACR assumption (for forward computations)
implement complex version of gfunc3_scale
WIP: finish scattering operator implementation
move forward_op_test.c to src; further minor changes