sci-mathematics/lrcalc: new package for Littlewood-Richardson coeffs.

sci-mathematics/lrcalc: new package for Littlewood-Richardson coeffs.

This package was imported from the sage-on-gentoo overlay with only a
few changes:

  * Updated from EAPI=6 to EAPI=7.
  * Enabled the test suite, which passes for me.
  * Changed the HOMEPAGE/SRC_URI to use HTTPS, and follow a redirect.
  * Updated LICENSE from GPL-2 to GPL-2+ based on the upstream
    LICENSE file.

The SageMath package can detect and use lrcalc from the system, so
having this package installed will help Gentoo users of SageMath avoid
pointless rebuilds of lrcalc.

Closes: https://bugs.gentoo.org/704572
Package-Manager: Portage-2.3.79, Repoman-2.3.16
Signed-off-by: Michael Orlitzky <mjo@gentoo.org>