Equality-constrained least-squares solver

[loiseaujc]

Following #1044, this PR implements an equality-constrained least-squares solver based on xGGLSE from lapack.

Proposed interfaces

• call solve_constrained_lstsq(A, b, C, d, x [, storage, overwrite_matrices, err])
• x = constrained_lstsq(A, b, C, d [, overwrite_matrices, err])

where A and b define the least-squares cost, while Cx = d define the equality constraints.

Progress

• Interface
• Base implementation
• Tests
• In-code documentation
• Specifications
• Example

Ping: @perazz, @jvdp1, @jalvesz