- 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

  - 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.

- added function get_variable_comparison to extract a variable from a list of musica netcdf output
- added function dygraph comparison to plot a comparison of musica output
- summary_markdown now also support a list of musica netcdf as input and then plot comparison graphics

added options to change the number of soil layer to be plotted

updated template options + add checks to summary function to skip graphic if variable does not exist