HistoricalLayer cannot handle all available BurnMD time formats

[afennelly-mitre]

In the current implementation of the HistoricalLayer class, we assume a fixed time format in BurnMD. However, the BurnMD data itself contains a variety of data formats. This was discovered as I was processing and analyzing the aggregated dataset.

The fix is simple - we just need to handle multiple time formats, rather than always assuming a single convention.