Add support for 18 OH in the CTP7 GE2/1 flavor

[Based on top of !170 (merged).]

As the title says... This MR aims at adding support for 18 OH on the CTP7 GE2/1 flavor in order to fully exploit the CXP transceivers.

It is composed of 2 commits:

1. The first commit unifies the number of supported OH to 16. Before this commit, registers and signals could support 12, 16, 24, or 32 OH. After this commit, all registers and signals support exactly 16 OH (except for the DAQ module which would otherwise see its changes be undone in the next commit). This is exploited by the CTP7 GE2/1 flavor.
2. The second commit increases the number of supported OH to 24, the maximum allowed by the data format without change. This is immediately exploited by the CTP7 GE2/1 flavor with support for 18 OH.

Tested on the production system at P5 during the GE2/1 trolley test and initial commissioning. Only the first commit is strictly necessary for operations at P5 whereas the second is there mainly for completeness and uniformity.