Filenames do not reflect month selections

[BSchilperoort]

In era5cli, the filename will contain the source (e.g. era5-land), the variable name, the year, if the data is hourly, and the extent (e.g. 3E-8E_50N-54N). However, the name does not contain the month(s). Users may choose to split requests up into single or multiple months per year to avoid making requests which are to large.

Currently this requires manually renaming the files after they have been downloaded, and before the next file is downloaded to avoid files being overwritten (see #76).

[BSchilperoort]

Additionally, having era5-land requests split up over months in the same way that requests are currently split up over years would allow users to much more easily download era5-land data, as you encounter a "request to large" error if you try to download a whole year.

E.g. the following request is not possible:

era5cli hourly --land --variables 2m_temperature  --startyear 2016 --endyear 2016 --levels surface --area 54 3 50 8

And users have to resort to this, where they manually go through months 1 -> 12.

era5cli hourly --land --variables 2m_temperature  --startyear 2016 --endyear 2016 --months 1 --levels surface --area 54 3 50 8