- default time_range to NULL in summary_markdown
- correctly transmit time_range argument in obs markdown template
- add some space

Updated summary_markdown to add observations

  - added automatic conversion for transpiration in MuSICA v2 output files
  - added get_variable_raw to avoid code repetition in get_variable

  - shiny app now use a separate css file
  - improved readability of the app. Graphics, regenerate and export button have now a fixed position. Left sidebar has now a scroll bar.

  in the correct order.