Update tutorial to use ERA5 data and jupyter notebooks

[sjavis]

I've updated the tutorial to use both ERA5 data and jupyter notebooks.

As with the old scripts, the notebooks can be run manually from the tutorial/ directory, but the outputs are also included in the documentation using nbsphinx (and nbsphinx_link to link to the the notebooks as they are outside the documentation folder). I added a tutorial group for the dependencies for running the notebooks - jupyter, matplotlib, cartopy.

The ERA5 data uses a single week of data (1950-09-01 to 1950-09-07) which includes about 4 active TCs. This takes much less time to download and preprocess (several minutes). I added tutorial/fetch_data.py to retrieve the data. It can do this in two ways:

• Download from CDS. This needs an account and API key.
• (default) Download from this github release I made.

To use the ERA5 data I also made some other changes:

• New preprocessing functions for the ERA5 data (these aren't strictly necessary here but they are helpful and will be needed down the line for batching):
  • calculate_norm_xy / calculate_wind_speed to get the wind speed from components.
  • multiply_field to multiply by a scalar - needed to convert geopotential to geopotential height or orography.
  • set_time_units to update the "days since YYYY-MM-DD" units as TSTORMS can't use negative time values.
  • squeeze_field to remove size-1 arrays as this needs to be done for some of the ERA5 data.
  • flip_axis to flip a coordinate direction. ERA5 latitude is decreasing but TSTORMS needs increasing.
  • set_netcdf_variable_name renamed to set_netcdf_info and made more general to add additional netcdf metadata.
  • The output file writing now specifies the netcdf4 backend. This is because in the latest version of cf-python the new default backend writes output that isn't compatible with netcdf-fortran (for TSTORMS).
• Fix a bug by changing the trajectories in Tempest Extremes to use the calendar from the input files rather than the more restricted calendar used in TempestExtremes itself.
• Fix the calendars in TSTORMS itself (Add compatibility with more CF calendar types TSTORMS#5).
• Update the TSTORMS input file guidance to describe newly discovered requirements.

This partially addresses #224.

[surbhigoel77]

LGTM