rems 0.7.0

• Added new helper function standardize_mdl_units() to detect when MDL_UNIT and UNIT are different, and convert METHOD_DETECTION_LIMIT to the same unit as RESULT (and update the MDL_UNIT column accordingly). (bcgov/wqbc#158, #57)
• Added new helper function lt_lake_req() that returns the requisition IDs of lakes in the B.C. long-term lakes monitoring program. To compliment this, filter_ems_data() now has a req_id argument that takes a vector of requisition ids (which can be supplied by lt_lake_req()). #60, @KarHarker
• lt_lake_sites() now only returns EMS IDs for active sites in the B.C. long-term lakes monitoring program. #60, @KarHarker
• Fixed a bug where the entire data cache was deleted when the historic data were updated. #62, #63