revert to previous style conventions for size_t constants

// i/o constants

const size_t colvarmodule::it_width = 12;

const size_t colvarmodule::cv_prec  = 14;

const size_t colvarmodule::cv_width = 21;

const size_t colvarmodule::en_prec  = 14;

const size_t colvarmodule::en_width = 21;

size_t const colvarmodule::it_width = 12;

size_t const colvarmodule::cv_prec  = 14;

size_t const colvarmodule::cv_width = 21;

size_t const colvarmodule::en_prec  = 14;

size_t const colvarmodule::en_width = 21;

const char * const colvarmodule::line_marker = (const char *)

  "----------------------------------------------------------------------\n";