Add option to disable precice-tools building

[davidscn]

Main changes of this PR

Introduces a cmake option to disable the building of precice-tools

Motivation and additional information

Building them might be undesired. The default flag (ON) delivers fully backward compatibility.

Author's checklist

• I added a changelog file with make changelog if there are user-observable changes since the last release.
• I ran make format to ensure everything is formatted correctly.
• I sticked to C++14 features.
• I sticked to CMake version 3.16.3.
• I squashed / am about to squash all commits that should be seen as one.
• Check handling of install targets

Reviewers' checklist

• Does the changelog entry make sense? Is it formatted correctly?
• Do you understand the code changes?

[fsimonis]

Is there any reason not to build it? Is has no additional dependencies and takes about a second to compile and link.

A user can always selectively compile libprecice using make precice.

[davidscn]

Is there any reason not to build it? Is has no additional dependencies and takes about a second to compile and link.
A user can always selectively compile libprecice using make precice.

Open for discussion, of course. I was not aware of a make precice option and I thought this is useful in case you develop and don't like to build precice-tools all the time. Makis was also asking today for this, which gave me the final decision to add something like this.

[fsimonis]

Still, it could come in handy in case the sym linking doesn't work or similar. So let's keep it.

[fsimonis]

I don't think this will need a changelog