Speed up mtop_util atom lookup

Speed up mtop_util atom lookup

The lookup of atom indices and properties on global atom index have
been sped up by moving functions to a new header file mtop_lookup.h
and by storing start and end global atom indices in gmx_mtop_t.
Another performance improvement is that the previous molblock index is
used as starting value for the next search.
The atom+residue lookup function now also returns the reside index.
This change also simplifies the code, since we no longer need a lookup
data structure.
A large number of files are touched because the t_atom return pointer
is changed to const also in the atomloop functions.

Change-Id: I185b8c2e614604e9561190dd5e447077d88933ca