Two time-related variables with identical name in MPADs

[Manuel Luque]

The user can create two time-related variables with identical name in MPADs.

To reproduce this issue: 1. Create a new MPAD. 2. Add a chance node, with name X. 3. Edit node X and set "Time Slice" to 0. 4. Right-click over "X [0]" and "Create node in next slice". 5. Edit node "X [0]" and set "Time Slice" to 1.

The user has two nodes with name "X [1]", which shouldn't be allowed.

[Miguel Ángel Artaso Landa]

The DistincVariableName constraint now checks that there are not two variables in the same time slice. Minor changes have been necessary in TimeSliceEdit and NodeDefinitionPanel classes to fix the issue.