Generalize multi-index set to support $\ell^p$ and not just $\ell^1$ thresholds

[JoWestermann]

Currently support multi-index sets are of the form $\Lambda_{\boldsymbol{k}, t} := \{\boldsymbol{\nu} \in \mathbb{N}^d_0 \ : \ \sum_{j=1}^{d} k_j \nu_j < t \}.$ The threshold condition $\sum_{j=1}^{d} k_j \nu_j < t$ can be replaced with a more general condition $\sum_{j=1}^{d} (k_j \nu_j)^p < t$ for $p > 0$.