[bugfix][gridgeometry] Always construct bboxtree and element map

Closes #1151

See merge request !3095

A thread-safe implementation with double-checked lock needs atomic load/store
operations to update the pointer to be thread-safe. Alternatively, a lock
has to be acquired every time when the function is called. Both implementations
impair the scalability of the multihreaded assembly (in the case of tpfa, or
any simulation that uses element maps or bounding box tree on an element level).

This changes the beahviour to always build the tree and the element map. This
usually occurs little runtime overhead in cases where the features are unused.
It is the most performant version as soon as one of the features is used.
There is however some memory overhead in the order of one or more ints per
element (seeds) and a bounding box hierarchy.

Bugfix/gridgeometry thread safe lazy initializer

Closes #1148

See merge request !3087

[cmake] install all modules

See merge request !3086

[doc] update license

See merge request !3088

Spatial parameters for Python bindings

Closes #1145

See merge request !3069

Ned Coltman authored 2 years ago and Timo Koch committed 2 years ago

* Update the generators for the new spatialparams classes
* use the new problem and spatialparams classes

[2p2c][doc] rewrite model documentation

See merge request !3056

[script/handbook] update release branch in script

See merge request !3082

[bugfix][parallel][solver] Rebuild parallel helpers after grid adaption

See merge request !3080

[tpfa][gg] Make sure hasBoundaryScvf_ is properly reinitialized in update

See merge request !3078

Provide defaults for FluxVariablesCache and FluxVariablesCacheFiller

Closes #867

See merge request !3077

The former checks whether a property is specialized for the given typetag,
The latter are convenience alias to get the defined type if it's
specialized for the TypeTag and some other given type if not.

[version] Bump version to 3.6-git

See merge request !3076

Feature/headercheck

See merge request !3064

Update changelog

See merge request !3073

[material][pnm] Fix position of deprecated attribute

See merge request !3071