einsum 0.1.0

Initial release of the einsum R package.

Features:

• einsum(): execute complex array multiplications using a concise and unambiguous based on the Einstein notation ("abc, b, c -> ac").
• einsum_generator(): generate an efficient C++ function that can be used for repeatedly executing the same multiplication. It can also produce the source code for a C++ function which you can integrate into your own package.

Thanks to @kokitsuyuzaki for helping me push this over the finish line and the numpy project for their equivalent function that inspired this package.