Add complex number support to linalg.pinv

[kgryte]

This PR

• adds complex number support to linalg.pinv.
• updates the allowed input data type from real-valued floating-point to any floating-point data type.
• makes explicit that, for matrix stacks, the pseudo-inverse must be computed for each matrix in the stack.
• adds an extended description helping contextualize the use rtol.
• (in practice) depends on Add complex number support to linalg.svd #561.

[rgommers]

This LGTM, thanks @kgryte