Use dash instead of underscore in variable name in file name

The filenames generated by `era5cli` look like this:
```
era5_temperature_1990_hourly.nc
era5_temperature_of_snow_layer_1990_hourly.nc
```
The usual convention for filenames, in e.g. the CMIP5 and CMIP6 projects, is to use underscores between the different facets and dashes if a space is needed inside a facet. This makes it easier to extract the different facets from the filename. Therefore, it would be more convenient if era5cli filenames looked like

```
era5_temperature_1990_hourly.nc
era5_temperature-of-snow-layer_1990_hourly.nc
```
because then people would be able to use a glob like
```bash
ls era5_temperature_*_hourly.nc
```
to find just the temperature variables instead of any kind of variable that starts with `temperature`.