Adds psalm and fixes first few findings

[NielsdeBlaauw]

No description provided.

[menno-ll]

psalm requires php 7.1 >=
We currently only define php 7.0 >=, so there is a change of errors occuring

see https://github.com/vimeo/psalm/blob/master/docs/running_psalm/installation.md

[menno-ll]

In the composer test script, testing with psalm takes a long time. We can make it faster for example if we only check changed files. maybe we should consider that. See command options with /vendor/bin/psalm --help or https://github.com/vimeo/psalm/blob/master/docs/running_psalm/command_line_usage.md

Command outputs:

➜  Clarkson-Core git:(feature/psalm) vendor/bin/psalm --threads=8 --diff --diff-methods --show-info=false
Scanning files...
Analyzing files...

------------------------------
No errors found!
------------------------------

Checks took **7.10** seconds and used 313.776MB of memory
No files analyzed
➜  Clarkson-Core git:(feature/psalm) vendor/bin/psalm --show-info=false                                  
Scanning files...
Analyzing files...

------------------------------
No errors found!
------------------------------

Checks took **13.76** seconds and used 361.078MB of memory
Psalm was able to infer types for 85.2657% of the codebase
➜  Clarkson-Core git:(feature/psalm) 

[menno-ll]

We can consider automatic fixing just like phpcbf that they have built in.

see https://github.com/vimeo/psalm/blob/master/docs/manipulating_code/fixing.md

[NielsdeBlaauw]

psalm requires php 7.1 >=
We currently only define php 7.0 >=, so there is a change of errors occuring

see https://github.com/vimeo/psalm/blob/master/docs/running_psalm/installation.md

Versie van psalm die we hier gebruiken v3.2.12. Die zou nog wel compatible moeten zijn met 7.0, maar gezien de wijzigingen met de #152 kunnen we van 7.1 uitgaan.

[NielsdeBlaauw]

We can consider automatic fixing just like phpcbf that they have built in.

Risky for now. Let's make a ticket and explore this later.