[ChinmayMoghe]

Improve efficiency seqmetrics instability index

Context

Issue #52

Under instability_index fn there is a String allocation to look up the weights,

for window in chars.windows(2) {
        let pair = format!("{}{}", window[0], window[1]);
        if let Some(val) = weights.get(&pair) {
            total += val;
        }

This could be avoided by not constructing a string as a key for weight lookup and rely on tuple of chars

What changed ?

load_instability -> Replaced key from String to (char, char), along with validation errors since if the key is invalid it's better to fail than have invalid / incomplete hash map for lookups

instability_index -> Removed format!() and replaced with (window[0], window[1]) for lookup

Tests

• Cargo test

[LCrossman]

this PR refactors for better error handling and improves efficiency with changing String allocation to a tuple. All tests pass and produce the expected output locally