Update profile processing routines

[mthielma]

Whats the purpose of this PR?

• Bug fix
• New feature
• Documentation update
• Other, please explain

Describe it in more detail below:
Profile processing required to merge the volume data sets before creating profiles, which was an issue when there were a lot of volume data sets. In this case, the processing became very slow. I added functions to circumvent this issue. Now the volume data sets can be passed as named tuples (in the same way as surface and point data is processed).
In addition, profile processing functions did not take into account topography data. I fixed that isse by adding the respective field in the ProfileData structure and by adding the topography data fields as input arguments to the respective functions.
I have started to add some tests. However, to test the topogrpahy, I'd need to load some topography data and I am not sure where this resides on the seafile folder.

Checklist

• The PR title is descriptive and starts with the appropriate tag: [BUGFIX], [ADDITION], [DOC], etc.
• New tests (either assessing the correct behaviour of new internal functions or the correctness of a tutorial) were added, or old tests were updated
• Affected tutorials have also been updated
• The new feature was added in a way that does not break public API
• New documentation related to the new feature was added
• The new code follows the contributor guidelines, in particular the Runic Style