add fill_in_missing_m_trend, get + fetch distinct
add fill_in_missing_m_trend, get + fetch distinct

- make get() and fetch() methods distinct from each other to allow
  for manual downloading of data from NDS2 when necessary for
  performance reasons (getting trends from frames is slow and
  sometimes impossible)
- add a tested version of fill_in_missing_m_trend() which will find
  missing m-trend data, fetch the corresponding s-trend data (which
  often does exist even when m-trend data fails to generate),
  calculate trend values, back up the original saved trends,
  check for missing values in the downloaded s-trends, replace
  the missing values, and finally saves the full downloaded m-trend
  data with missing data inserted.

stefco/geco_data,stefco/geco_data,stefco/geco_data