Separate pgr_types.h & create new file path_element_t.h

[XJTUmg]

• Delete struct path_element_t from pgr_types.h.
• Create new file path_element_t.h.

[cvvergara]

looks like this structure is not used any more in the code:
path_element_t;

git rm  include/c_types/path_element_t.h
git commit -a -m 'removing unused structure`
git push