[my_em.py] maximize_em_one

- this module, previously named maximize_em_salpeter,
  has been generalized for any of salpeter, chabrier,
  and kroupa imftypes.
- instead of alpha_fit, parameter that is varied is
  now referred to as param, e.g. alpha_fit -> param_fit.
- for imftype = 'chabrier' / 'kroupa' one parameter is
  passed as array (param_arr), the other must be passed
  as a **kwarg (see if statements in lines 38+)