add contrasts property to reneeDataSet

[kelly-sovacool]

• Property to store contrasts as model formulae for DESeq2.
• Also need to let users specify which factor level should be first for each column (e.g. wildtype before knockout).

[kelly-sovacool]

look into how DESeq2 handles the design: https://github.com/thelovelab/DESeq2/blob/4497a51ab22e86513ebaec930de3825b45fc89a4/R/AllClasses.R#L25-L59