Remove variable subscript affectations

This means that one cannot write anymore things like
```
x[0] = 1;
```
or
```
A[0,0] = 1;
```
where x is an R^d and A an R^dxd.

This is done for these reasons:
- first these kind of instructions are more or less useless.
- Second they do not follow the philosophy of pugs since they give the
sensation that one could modify specific values of discrete functions
for instance, which should never enter the language
- Finally, getting ride of these constructions permits to ensure that
all variables in a pugs' script are properly initialized

Fixes #28