MVM: Analyze MVM WHT array for Q&A

[stscijgbot-hstdp]

Issue HLA-740 was created on JIRA by Warren Hack:

Statistics of the weight map could also be generated for each layer as a way to identify any areas where exposures with much lower exposure time (and therefore, higher photometric uncertainties) have been included.

Statistics may include a simple histogram of exposure times for the exposures in each layer or even for the entire SkyCell.  

Stats on percent of exposed pixels with each level of exposure time may also be useful.  The 'level' would be based on the average exposure time and broken down into groups of exposure times using algorithms such as KMeans as already implemented in 'poller_utils' for defining the exposure time layers.  

Implementing a means of visualizing these results will be part of this ticket since the interpretation of the results requires knowledge of how the results are generated and stored.