Adding missing operations

[Ovascos]

Adds missing operations on the C++ interface:

• make Context copy-able and a wrapper constructor for lp_polynomial_context_t
• direct constructor for DyadicInterval(long) to avoid ambiguous implicit constructor calls
• missing constructor DyadicRational(long), since DyadicRational(int) is present
• Operators ==, !=, <, <=, >, and >= between Integer and long to avoid unnecessary implicit construction of Integer objects
• Operators +=, -=, and *= as well as ==, !=, <, <=, >, and >= between Polynomial and Integer to avoid unnecessary construction of Polynomial objects with potentially incorrect context (partially fixes Operators are not context aware #97)

[daniel-larraz]

After these changes, creating a constant polynomial like this:

poly::Polynomial p1 = poly::Polynomial(ctx, 1);

no longer works. The integer value now needs to be wrapped explicitly:

poly::Polynomial p1 = poly::Polynomial(ctx, poly::Integer(1));

I'm not sure this is the intended behavior.

[Ovascos]

I'm not sure this is the intended behavior.

Of course not. Fixed.

[Ovascos]

The last commit added explicit to the constructor of Variable which might be a braking change. Let's wait some days for user feedback before merging.