Implement paired t-test

[macovskym]

To use a paired t-test, you must do the following:

1. Create a CSV file for each experiment with the format:
   • First row containing the names of treatments
   • First column containing names for the pairings (can be empty)
   • Cells containing the names of samples as used in the experiment data tables

For example:

	0wk	4wk	8wk
mouse1	sample4	sample48	sample74
mouse2	sample9	sample34	sample71
mouse3	sample14	sample38	sample65

2. Add the paths to the pairing CSV files to metadata.json:

   ...
   "experiments": [
   {
       "name": "experiment1",
       ...
       "pairingsFile": "experiment1_pairings.csv"
   },
   {
       "name": "experiment2",
       ...
       "pairingsFile": "experiment2_pairings.csv"
   },
   ...
   ],
   ...

3. Specify paired t-test in the config file:

   {
   ...
   "differenceMethod": "pairedttest",
   ...
   }

[bruceamm]

Allows user to calculate paired t-test, verified output in Excel (no errors or issues, AB/IS)