Rectified MVE class

[dannys4]

Exactly what it sounds like; a class, as described in previous PRs, to define a function $T:\mathbb{R}^d\times\mathbb{R}\to\mathbb{R}$ which performs
$$T(\mathbf{x},y) = \sum_{\vec{\alpha}^1_x} c^1_{\vec{\alpha}^1_x} \Psi_{\vec{\alpha}^1_x}(\mathbf{x})+ \sum_{\vec{\alpha}^2}c^2_{\vec{\alpha}^2}r\left[\Psi_{\vec{\alpha}^2_x}(\mathbf{x})\right]g_{\alpha^2_y}(y),$$

where $\vec{\alpha}^1_x$ and $\vec{\alpha}^2$ do not necessarily come from the same multi-index set (indeed, we want all $\vec{\alpha}^2$ to come from a set that is exclusively non-constant in $y$ to avoid redundant terms); further, all ${g_j}$ should be monotone, and $r:\mathbb{R}\to\mathbb{R}^+$ should probably be bijective (not sure how necessary that is though), and the second set of coefficients, ${c^2_{\vec{\alpha}_2}}$, must be nonnegative.

This went through several iterations and I think this is the least bad choice here. The provided MVE would've worked to some degree, but defining the LogDet and Inverse functions would've been invasive, and we'd lose some efficiency from spawning more parallelism than necessary (as well as the fact that MVE is engineered to give many outputs simultaneously, whereas this is a special case).

Bindings and a MapFactory method will be created in a future PR.

[mparno]

Thanks Danny! I have one comment about the use of virtual keywords, but everything of real substance looks great! It'll be great to have this in MParT.

[mparno]

While not strictly necessary, I would advocate for keeping the virtual keyword across all of the *Impl classes. It avoids some compiler warnings and, even though we have the override keyword, makes it more obvious that these functions stem from the base class.

[dannys4]

I'm not sure what compiler warnings this avoids, as any non-final function that overrides is, by definition, a virtual function (I haven't bothered to look it up in the actual standard, but it's on the cppref virtual site). That being said, the difference is minimal and I'm more than happy to undo this change.

[mparno]

Thanks Danny!