Unit tests for `calculate_variable_percentile_from_value()` and `calculate_multiple_variable_percentiles_from_values`

This MR addresses: #55 (closed)

It creates a simple percentile df and tests calculation of variable percentiles using an input flow value and a month_day value in the format MM-DD. It also uses a simple flow df to test calculation of multiple variable percentiles from values. On the latter test of multiple variable percentiles, it also checks df metrics like shape and index values.

Please let me know if you can think of any other unit tests to add. One thought I had is: what if min and max are not in the percentile df? Do you think I should add that scenario too? Thanks!