NPI-3676 Exception handling on (unsupported) SP3 velocity data output

[treefern]

gnssanalysis currently does not correctly support output of SP3 velocity data.

At present, currently calling gen_sp3_content() on a DataFrame containing velocity data, will result in a non-compliant SP3 file with velocity data in additional columns (not in additional, interlaced rows as the spec calls for).

This PR adds a check for velocity columns. If any velocity columns are found a warning is logged, the columns are deleted, then output continues as normal on just the position data.

Optionally based on a new parameter to gen_sp3_content(), a NotImplementedError can be raised instead of logging a warning.

[treefern]

A small fix for the sp3_merge() unit test came along for the ride.
It turns out that pyfakefs actually uses a real temporary directory under the hood. It seems this can lead to data persisting between unit test runs!

[ronaldmaj]

This is a nice clean-up PR and sets up a new unit-test for future. Happy for this to go in

[ronaldmaj]

All good from what I can see after the merge with main - good to go