Implement dashed varname option

[BSchilperoort]

Based on #53, I've added the option to have the output variable name underscores replaced with dashes (--dashed-varname).

For example
era5_temperature-of-snow-layer_1999_hourly.nc instead of
era5_temperature_of_snow_layer_1999_hourly.nc.

I am not sure if we should add this (now), because there are more and more optional arguments, but it is quite useful, as @bouweandela outlined in the issue.

I would not be against making this the default in the future.

[codecov]

Codecov Report

Merging #148 (5ef186e) into main (e6c85c4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

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  Misses          2        2           

[Peter9192]

We just discussed this and I agree on the general outline. Let's update #53 with a summary of the main pro's and cons of making this the default, so we can have a focused discussion on the long-term behaviour.

[BSchilperoort]

Note that I have decided to change the argument to --dashed-varname instead of --dashed_varname, after doing some more research on what is more commonly used/better.