WIP: dtopotools status update

WIP: Do not merge yet.

@mandli: We wanted to get 5.2.0 out so this hasn't been included.  The current examples/tsunami/chile2010 still uses the old *okada2.py* rather than *dtopotools*, so it's not being used in the core code.

There are some things to discuss still. In particular:

* SubFault parameters *dimensions* and *coordinates* are tuples *(length,width)* and *(longitude,latitude)* respectively, which other parameters such as *slip* are scalars.  I would suggest we simplify by replacing these tuples with separate parameters *length*, *width*, *longitude*, and *latitude*.  This will simplify some of the logic and also would be less confusing I think than having to remember what order they come in, particularly since subfault files might list them in various orders in their columns.

* I want to re-add a *defaults* parameter when reading a subfault file that specifies a default value for parameters not listed in each row of the subfaults file, e.g. NOAA SIFT format for "unit sources" always has *length=100, width=50* and these aren't listed.  It could be that somebody's format has a default *length* but a different *width* for each subfault (another reason for the first suggestion above).

* *coordinate_specification == 'noaa_sift'* should be added in general (I started to do this but ran into the above issue).

* Time-dependent fault motions still need to be tested.

* Plotting of subfault slip, depth, etc. has to be re-added.

* @property functions need to be added.

* We should add a dtopo_type for static deformation that only lists a single time and the deformation, rather than listing a full array of 0's at t=0 and then the deformation at a later time, as currently done with all types. This used to exist but got lost somewhere along the way.

* A full test suite is needed, and more examples for users.