xgeometry module started

- xgeometry module contains all breakable geo. quantities. now only
xedge is implemented

- About derived data type and structure constructor:
* gfortran in Centos support allocatable arrays as components of a
derived data type
* structure constructor can directly allocate the allocatable array
components according to your input dimension and values of arrays; it
also overwrites the 'prepared' allocated dimension and values, if
mismatched