Separate 2d contact code to own package (#195)

Moved plane contact related stuff to own separate package
`MortarContact2D.jl`, where the development continues.

The following changes to test files are done:

1) Problem name for plane mortar coupling is `Mortar2D` (was `Mortar`
before), and later on 3d coupling will be `Mortar`. So the dimension
of coupling operator is explicitly given in a problem name.

2) Before elements to coupling was defined using
```julia
update!(problem.elements, "master elements", master_elements)
add_elements!(problem, [slave_elements; master_elements])
```
Now, explicitly give master and slave elements as
```julia
add_slave_elements!(problem, slave_elements)
add_master_elements!(problem, master_elements)
```
Keep on mind that Lagrange multipliers are in slave side.

src/problems_mortar.jl

@@ -66,6 +66,11 @@ function assemble!(problem::Problem{Mortar}, time::Float64)
     assemble!(problem, time, dimension, use_forwarddiff)
 end
 
+function get_slave_elements(problem::Problem)
+    cond(el) = haskey(el, "master elements") || haskey(el, "potential master elements")
+    return filter(cond, get_elements(problem))
+end
+
 """ Given a CCW ordered set of vertices, calculate area of polygon.
 
 Examples
@@ -99,7 +104,7 @@ function diagnose_interface(problem::Problem{Mortar}, time::Float64)
     field_dim = get_unknown_field_dimension(problem)
     field_name = get_parent_field_name(problem)
     slave_elements = get_slave_elements(problem)
-    
+
     I_area = 0.0
 
     if props.split_quadratic_slave_elements
@@ -125,7 +130,7 @@ function diagnose_interface(problem::Problem{Mortar}, time::Float64)
         info(repeat("-", 80))
         info("Processing slave element $(slave_element.id), type = $(get_element_type(slave_element))")
         info(repeat("-", 80))
-    
+
         S_area = 0.0
         S_area_in_contact = 0.0
         for ip in get_integration_points(slave_element)
@@ -245,7 +250,7 @@ function diagnose_interface(problem::Problem{Mortar}, time::Float64)
         I_area += S_area_in_contact
 
     end # slave elements done, contact virtual work ready
-    
+
     info("Area of interface: $I_area")
     info("Smallest cell area: $(minimum(C_areas))")
     info("Smallest polygon area: $(minimum(P_areas))")