Skip subsampling if batch size is less than n_samples_subsample (#707)

Includes a check that n_samples_subsample is less than the batch size - i.e. length(samples). If this is the case, the subsampling task RandomSubsampleStringArray is skipped. 

Also updates documentation to indicate this.

inputs/templates/terra_workspaces/cohort_mode/cohort_mode_workspace_dashboard.md.tmpl

@@ -166,7 +166,7 @@ Read the full EvidenceQC documentation [here](https://github.com/broadinstitute/
 
 Read the full TrainGCNV documentation [here](https://github.com/broadinstitute/gatk-sv#gcnv-training-1).
 * Before running this workflow, create the batches (~100-500 samples) you will use for the rest of the pipeline based on sample coverage, WGD score (from `02-EvidenceQC`), and PCR status. These will likely not be the same as the batches you used for `02-EvidenceQC`.
-* By default, `03-TrainGCNV` is configured to be run once per `sample_set` on 100 randomly-chosen samples from that set to create a gCNV model for each batch. If your `sample_set` contains fewer than 100 samples (not recommended), you will need to edit the `n_samples_subsample` parameter to be less than or equal to the number of samples.
+* By default, `03-TrainGCNV` is configured to be run once per `sample_set` on 100 randomly-chosen samples from that set to create a gCNV model for each batch. To modify this behavior, you can set the `n_samples_subsample` parameter to the number of samples to use for training.
 
 #### 04-GatherBatchEvidence
 

wdl/TrainGCNV.wdl

@@ -112,7 +112,7 @@ workflow TrainGCNV {
     }
   }
 
-  if (defined(n_samples_subsample) && !defined(sample_ids_training_subset)) {
+  if (defined(n_samples_subsample) && (select_first([n_samples_subsample]) < length(samples)) && !defined(sample_ids_training_subset)) {
     call util.RandomSubsampleStringArray {
       input:
         strings = write_lines(samples),