Changed procedure table to use_case, changed samples date field to datetime

"procedure" is a keyword, so it's better to use a non-reserved word for a table, otherwise you always have to use the   slanted quote mark around the table everytime you use it.  Also, we discussed this in our standup yesterday and decided that u"use case" was a better description anyway.  I also changed the samples.sample_date field to a datetime field so that we can have multiple entries for a single day.