chore: change point_type to point_type_t and use 'using' at those places

[barendgehrels]

This PR

• changes typename point_type<X>::type to point_type_t<X>
• at those places, use using i/o typedef
• in lines immediately before / after that, also use using
• sometimes also change to coordinate_type_t there (but that is not consistently)

I also verified clang-tidy with its usage modernize-use-using but that is not giving the desired results at this moment.

Clang tidy can, with its option modernize-use-using, replace typedef with `using. It sometimes works well, but:

• it can replace typedef differential_quantities<CT, EnableReducedLength, EnableGeodesicScale, 1> quantities with using quantities = int which is plainly wrong
• it can replace typedef typename boost::range_value<Range const>::type point_t with using point_t = typename boost::range_value<const Range>::type which moves the const to left, and that is not what is in our conventions
• it removes any newline and changes neatly formatted long typedefs into a very long one-lined using clause.

Therefore I decided to not use it. So this commit changes to point_type_t, plus using. A next might change to coordinate_type_t plus using and then we can change the rest manually, or semi-automatically (still using clang-tidy but judge per change)

[awulkiew]

It looks good. Thanks.

[vissarion]

LGTM, thanks!