refactor(core): Simplify detrend process

WHY IS THIS CHANGE NEEDED:

• logical steps of computation unclear, therefore unclear if result of computation is what we want

HOW DOES THE CHANGE SOLVE THE PROBLEM:

• replace moving_average() and lines 74-76 with pandas.DataFrame methods -- simplifies code; standardised routines
• rename variables so they are more informative

EVIDENCE THAT COMMIT WORKS:

• generate random DataFrame, original & new implementation produce same outputs

REFERENCES:

• Issue #21 (closed)

assigned to @amuoz

approved this merge request

mentioned in commit 3e580784

merged

mentioned in commit aliby@f74f8944