Fixes for reading and writing 3D (grid) data sets

[drroe]

Addresses #623 and adds some enhancements for reading/writing 3D data.

• 3D data sets written in "standard" format can now be read back into cpptraj in either floating point or double precision.
• 3D data sets written in "standard" format now contain dimension, origin, and spacing information.
• Bin (voxel) X Y Z coordinates are now always correct - previously did not correctly take into account origin or step, and were completely wrong for non-orthogonal grids.
• Bin coordinates for non-orthogonal grids are now correctly adjusted when origin is not 0,0,0.
• 3D data sets written in "standard" format can now be written as "sparse" (i.e. empty voxels are not written) - this can provide significant size savings.

Adds the following syntax for 3D "standard" reads:

read3d:      Read data as 3D grid. If no dimension data in file must also
             specify 'dims'; can also specify 'origin' and 'delta'.
	dims <nx>,<ny>,<nz>   : Grid dimensions.
	origin <ox>,<oy>,<oz> : Grid origins (0,0,0).
	delta <dx>,<dy>,<dz>  : Grid spacing (1,1,1).
	prec {dbl|flt*}       : Grid precision; double or float (default float).
	bin {center|corner*}  : Coords specify bin centers or corners (default corners).

Adds the following syntax for 3D "standard" writes:

nosparse   : Write all 3D grid voxels (default).
sparse     : Only write 3D grid voxels with value > cutoff (default 0).
	cut : Cutoff for 'sparse'; default 0.

Also adds several tests.