Added group option and shift command

README.md

@@ -153,4 +153,23 @@ This option deletes vacancies on a plane which when minimized should result in a
 
 `bx by bz` - The burgers vector for the dislocation
 
-`poisson` - Poisson ratio for continuum solution
+`poisson` - Poisson ratio for continuum solution
+
+### Option Group
+
+`-group select_type group_shape shape_arguments additional keywords`
+
+This option selects a group of either elements, nodes, atoms and applies some transformation to them.
+
+`select_type` - Either `nodes`, `atoms`, `elements`, `nodes/atoms`, `all`. When using the option `nodes` only nodes which are within the group are selected, `elements` selects elements based on whether the element center is within the group, `nodes/atoms` selects both nodes and atoms for the group.  `all` selects elements based on the element center and atoms based on their position.
+
+`group_shape` - Specifies what shape the group takes and dictates which options must be passed. Each shape requires different arguments and these arguments are represented by the placeholder `shape_arguments`. The accepted group shapes and arguments are below:
+
+*Block:*
+
+`-group nodes block xlo xhi ylo yhi zlo zhi`
+
+This selects a group residing in a block with edges perpendicular to the simulation cell. The block boundaries are given by `xlo xhi ylo yhi zlo zhi`. 
+
+`additional keywords`- Represents the various transformations which can be performed on a group. These additional keywords are given below.
+