Implementation of linear combinations
First version of LC inner products
Remove overlap matrix in LCWP

We can now compute this matrix easily with new inner products.
Implement clone
Wavepackets know how to compute their gradient
Improve linear combinations
Review and document inner products of linear combinations
Review and document observables of linear combinations
Fix missing block selection in evaluation scripts
Helper functions for loading and saving whole wavepackets at once.
Simplify file creation with IOM, drop parameters input.
Small improvments in the IOManager class.
First half of new IOM plugin for linear combinations
Fix data type inconsistency for fresh wavepackets
Save and load wavepackets from arbitrary datablocks
Implement dump of wavepackets of a linear combination
Prevent from adding the same wavepacket multiple times to the same linear combination

If we would allow this, it could lead to errors when saving linear combinations.
Note that we only disallow to add the same *instance* twice. Adding multiple
different instances which are only mathematically identical is no problem.
Implement load and save of full linear combination objects
Fix doc build error
Implement very generic (but inefficient) wavepacket loader/saver