removing redundant variables and constraints

[aivarsoo]

Minor fixes to the example:

Readme modifications:

1. Comment added on bins.lowest_num_bin
2. A width constraint is modified and commented on. Now the text in readme corresponds to the code

Code modifications:

1. Without loss of generality we can assign the first case to the first bin, therefore the variable bin_loc[0,0] is fixed to be equal to 1.
2. Since the minimum number of bins is estimated and saved in bins.lowest_num_bin, we can fix the first bins.lowest_num_bin bin_on[j] variables to be equal to one. Thus we can remove bins.lowest_num_bin variables, as well as, 2 * bins.lowest_num_bin constraints.
3. The width constraints are now added only once
4. The length constraint for j=0 is not add, since it is redundant.
5. All the inequality constraints are changed to have be lower or equal than <= for consistency.
6. The order of adding the constraints is modified so that the add_discrete are added first.