The `core-metrics-phylogenetic`

cli help docs in 2020.8.0 state:

```
Outputs:
--o-rarefied-table ARTIFACT FeatureTable[Frequency]
The resulting rarefied feature table. [required]
--o-faith-pd-vector ARTIFACT SampleData[AlphaDiversity]
Vector of Faith PD values by sample. [required]
--o-observed-features-vector ARTIFACT SampleData[AlphaDiversity]
Vector of Observed Features values by sample.
[required]
--o-shannon-vector ARTIFACT SampleData[AlphaDiversity]
Vector of Shannon diversity values by sample.
[required]
--o-evenness-vector ARTIFACT SampleData[AlphaDiversity]
Vector of Pielou's evenness values by sample.
[required]
--o-unweighted-unifrac-distance-matrix ARTIFACT
DistanceMatrix Matrix of unweighted UniFrac distances between
pairs of samples. [required]
--o-weighted-unifrac-distance-matrix ARTIFACT
DistanceMatrix Matrix of weighted UniFrac distances between pairs
of samples. [required]
--o-jaccard-distance-matrix ARTIFACT
DistanceMatrix Matrix of Jaccard distances between pairs of
samples. [required]
--o-bray-curtis-distance-matrix ARTIFACT
DistanceMatrix Matrix of Bray-Curtis distances between pairs of
samples. [required]
--o-unweighted-unifrac-pcoa-results ARTIFACT
PCoAResults PCoA matrix computed from unweighted UniFrac
distances between samples. [required]
--o-weighted-unifrac-pcoa-results ARTIFACT
PCoAResults PCoA matrix computed from weighted UniFrac
distances between samples. [required]
--o-jaccard-pcoa-results ARTIFACT
PCoAResults PCoA matrix computed from Jaccard distances between
samples. [required]
--o-bray-curtis-pcoa-results ARTIFACT
PCoAResults PCoA matrix computed from Bray-Curtis distances
between samples. [required]
--o-unweighted-unifrac-emperor VISUALIZATION
Emperor plot of the PCoA matrix computed from
unweighted UniFrac. [required]
--o-weighted-unifrac-emperor VISUALIZATION
Emperor plot of the PCoA matrix computed from
weighted UniFrac. [required]
--o-jaccard-emperor VISUALIZATION
Emperor plot of the PCoA matrix computed from
Jaccard. [required]
--o-bray-curtis-emperor VISUALIZATION
Emperor plot of the PCoA matrix computed from
Bray-Curtis. [required]
```

Why do all of these have to be required? Maybe the user doesnâ€™t want all of that output. How about changing all of these to option, and if the user doesnâ€™t provide a path, then that analysis isnâ€™t done and no file is written.