Inconsistent indexing for initial variable bounds

It appears that the `s.variables[i].toInd` value is computed differently for the first variable created (i==0) than it is treated for variable created later.

The indexing for the i=0 variable goes from `fromInd==0` to `toInd==len(var)-1`, while for a subsequent variable where the indexing goes from `fromInd=j` to `toind==j + len(var)`

Example:

```
import numpy as np
from cylp.cy import CyClpSimplex

s = CyClpSimplex()
x = s.addVariable('x', 3)
y = s.addVariable('y', 2)
s += 1 <= x <= 2
s += 3 <= y <= 4

print(s.variablesLower[s.variables[0].fromInd: s.variables[0].toInd])
print(s.variablesLower[s.variables[1].fromInd: s.variables[1].toInd])

[Out:]
[1. 1.]
[3. 3.]
```

Even more succinctly, this behavior is immediately visible on initializing the initial variable:
```
s = CyClpSimplex()
x = s.addVariable('x', 3)
print(s.variablesLower[s.variables[0].fromInd: s.variables[0].toInd])
```

I would expect there to be three entries for the bounds on x.