Metric extensions

[malarbol]

Built on top of #1738

This PR introduces the type Metric-Extension P M between a pseudometric space P and a metric space M as the type of isometries from P to M.

[fredrik-bakke]

What you call a Metric-Extension should really be called something of the form is-metric-extension. A metric extension of P would be a choice of a metric space M together with an isometry. I.e., the M is bundled with the isometry. I am still entirely unconvinced about the terminology choice of Metric-Extension, though, but I haven't come up with a better name which is why you haven't heard from me.