Minor bugfixes - plastid tRNAs, tRX cluster parents, DESeq heatmaps

Bugfixes

• When specifying custom input fasta sequences, plastid tRNAs are now specified as a separate file and handled independently from mitochondrial sequences
• If a cluster with a "tRX" parent sequence is unsplit from a "tRNA" child, naming conventions for this composite cluster are now correct
• When a heatmap of differentially expressed isodecoders or anticodons only has one sequence, the "basemean" for the sequence in the right most pane of the heatmap is given as a bar plot, not a line plot. Avoids errors in trying to make line plots from one data point.