Add angle conversion factors, and include typed constants.

Add DEG_TO_RAD and RAD_TO_DEG constants. Since these constants
are often used in rendering (single precision floats) and
coordinate calculations (double precision floats), add double,
float, and Real versions of the constants to reduce the
number of static_casts in working code.

Change-Id: I11861a4fee9a363cd3dfb6cac59c825df22b148c
Add crystal support to the cjson format.

Adds a new top-level member for the cell specification
(named "unit cell"), and a sibling of atoms::coords:3d named
"3d fractional".

Change-Id: I1589416fd9d114a8973c4ed17380fa316adbbdd9
Merge remote-tracking branch 'origin/master' into cjson-crystal

Conflicts:
	avogadro/quantumio/molden.cpp

Change-Id: Ib0bffc6be0ebd0722250e265f463f0320a02bd8c
Merge topic 'cjson-crystal' into master

47faf6e2 Merge remote-tracking branch 'origin/master' into cjson-crystal
f6c24bae Add crystal support to the cjson format.
59adcba8 Add angle conversion factors, and include typed constants.