Simplify the calculator logic and de-couple from input data structures

[gassc]

Currently the calculator expects a 1D array of precip estimates values and a corresponding header array of the same length, where the array position in each corresponds (among other things). The values are read into a NumPy array and all subsequent calculations are effectively batched over each value in the array. This is a holdover from the original codebase and perhaps a bit more complicated than necessary. It makes running an individual calculation needlessly compex.

The core calculator logic should operate just on

• a precip value,
• a flow length
• an average slope
• a basin area.

To run on multiple storm event frequencies, we run the calculation in a loop. It's not a complicated or long running equation--just some pretty basic math--so efficiency isn't really an issue here.

Anything related to data structures, reading/writing to a table, etc., should be handled outside of the core script.

[gassc]

Started to address in 39162ba