Add more general linear operator constraint to sdp

[SuvarshaChennareddy]

Hello everyone, This is related to issue #72. I've allowed the ability to store constraints as std::function objects. I have not modified/added tests yet. However, if this PR looks alright, I'll be willing to do so. Any comments would be greatly appreciated.

[mlpack-bot[bot]]

Thanks for opening your first pull request in this repository! Someone will review it when they have a chance. In the mean time, please be sure that you've handled the following things, to make the review process quicker and easier:

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Thank you again for your contributions! :+1:

[SuvarshaChennareddy]

Will do! Thank you!

[mlpack-bot[bot]]

This issue has been automatically marked as stale because it has not had any recent activity. It will be closed in 7 days if no further activity occurs. Thank you for your contributions! :+1: