resampling algorithms are now adjustable in COREG; bugfix for not respecting taking nodata value when de-shifting

 COREG:
 - added keywords 'resamp_alg_deshift' and 'resamp_alg_calc'
 - updated docstring
 - added warning when resampling algorithm 'average' is chosen
 - _calc_shifted_cross_power_spectrum(): bugfix for not using pyfftw even if available
 - refactored calc_subpixel_shifts() to _calc_subpixel_shifts()
 - added _dict_rspAlg_rsp_Int

 DESHIFTER:
 - refactored dict_rspAlg_rsp_Int to property _dict_rspAlg_rsp_Int
 - added 'nodata' keyword

 MAIN:
 - fixed some typos