Variable is not context aware

[Ovascos]

The C type lp_variable_t is an integer that is valid with one variable_db_t. However, in its C++ wrapper code, the Variable class is not aware of the context and thus the variable DB it belongs to.

This leads to a failing assertion in the following code:

Context ctx;
Variable x(ctx, "x");
Polynomial p1(ctx, x);
p1 *= x;

This is because the last line implicitly calls the constructor Polynomial(x) which is incorrect, as x actually belongs to ctx instead of the default context. An explicit construction with p1 *= Polynomial(ctx, x) is required instead.

Another problem occurs when indexing invalid memory in variable_db_t:

Context ctx;
Variable x(ctx, "x");
Polynomial p(x);
p *= x;

Here x is allocated in ctx's variable DB, while p is created in the default context. This leads to an out-of-bounds access in the default context's variable_db_t as the variable x was created there. No assertion fails in this case.
It would be expected that p1 is created in ctx instead of the default context.