Resolve "Write text for THRILL page"

Closes #38

See merge request !9

Resolve "Beam splitter: Check splitting ratio"

Closes #40

See merge request !6

Add unit tests.

Resolve "Implement "inverted" feature for group nodes"

Closes #14

See merge request !5

The original graph is kept untouched.

Update names of components in filter_test

Resolve "Add detector node "spectrometer""

Closes #30

See merge request !4

This is only a preliminary solution until we have a general reporting.

Copied from EnergyMeter. Needs modifications yet.

Resolve "Add detector node "Energy meter""

Closes #29

See merge request !3

Update docs. Update filter_test example.