CLI for exporting waveforms

[SBlokhuizen]

Note: this PR is build on top of the implementation of the WaveformExporter (#29)
This PR introduces a first draft for the CLI to call the exporter methods described in the WaveformExporter. The user can choose between 3 methods of determining the time arrays:

• If no time option is given, the time points of the minimal representation are used.
• if --num_interp is given, the waveform will be linearly interpolated between its start and end time, using this many points. For example, to generate the waveform using 5 points:
  waveform-editor export-ids ./test_yaml.yaml "imas:hdf5?path=./test_equilibrium#equilibrium/time_slice()/boundary/elongation" --num_interp 5
• If --times is provided, the user can point to a CSV file, containing the time steps, which are expected in a column-based format. For example, create a times.csv containing:

0
1
5
7
12
12.5
13
13.1
14

These can then be provided using:
waveform-editor export-ids ./test_yaml.yaml "imas:hdf5?path=./test_equilibrium#equilibrium/time_slice()/boundary/elongation" --times times.csv
Note: there are no tests for the CLI yet, these can be made once we settle on a syntax for it.

[maarten-ic]

Let's first align how the export should work, see #29

[SBlokhuizen]

The functionality for exporting through a CLI is moved to #45