Add Endpoint to compute Chart Timestamps

Currently the timestamp values of a chart are computed by offsetting all the timestamp by the datetime at the start of the file. To account for discrepancies between the timestamps and gps dmyhms. This should be fixed by adding a new endpoint to the backend which explicitly calculates the timestamps of the file in datetime with a higher number of references to the gps time. The goal of this change is to hopefully better account for errors in the teensy timestamp.

Start by locating the backend code that currently offsets chart timestamps using the datetime at the start of the file, then trace how GPS dmyhms values are read. Define the new endpoint around explicit file timestamp calculation with more GPS-time references, and verify that it better accounts for Teensy timestamp errors.