fix color.by bug and introduce summarise.by feature for geyserEnrichment

[toobiwankenobi]

Hi @ncborcherding,

This turned out to be a bigger adventure than originally expected. I'm sorry that this pull request now introduces a lot of changes. I am aware that this is not ideal to review.

• color.by bug covered in issue color.by argument does not always work #163
• implement summarise.by argument to summarise/collapse data by meta data column proposed in issue feature request: summarise.by for geyserEnrichment #164
• formatting of help section of two functions was not properly displayed, fixed with commit 4f9de37
• Commit d021190 moves summary helper function to utils.R since it's now used by two functions
• enables scaling if color.by is another feature (i.e. gene set) in geyserEnrichment()

Best wishes,
Tobi

[ncborcherding]

@toobiwankenobi

No problem at all - I appreciate all the help and things look great!