Fix for issue #125.

The current implementation does not take into account the prvalue that
creates an issue while storing it. The prvalue is destroyed after the
end of its scope, and if you try access it, you are entering into
undefined land of C++.

To solve this issue, we employ the trait `std::is_lvalue_reference` to
determine if need to extend the lifetime or not. If extension is needed,
we store the value with the help of universal forwarding. Otherwise, we
extend lifetime using `const &`.

Decoupled each `constexpr if` conditions into function using requires
clause, which cleaned-up the compare function. Also, added the check for
static extents that removes the runtime check.

Since the patch is fixing the lifetime of the tensor, we cannot combine a prvalue with the old ublas expression. Therefore, we have to bind the prvalue with the name to increase the lifetime.

This function casts any `tensor_expression` to its child class, and it
also handles recursive casting to get the real expression that is stored
inside the layers of `tensor_expression`.

Previously, we would create a new tensor from the expression template
every time there is a comparision. Also, fixed the spelling to
`cast_tensor_expression` from `cast_tensor_exression`.

… undefined behaviour

…between different extents types

…bjects to catch type errors

…ove redundant casting

The standard does not define the complex class other than float, double,
and long double. It leaves the implementation on the hands of vendors,
and it could lead to the portability issue or undefined behaviour, which
is not warranted.

…aintainability