[ENHANCEMENT] Q vector generation: load a user-provided array

**Is your feature request related to a problem? Please describe.**
Some users dealing with crystalline systems have measured inelastic neutron scattering data at specific values of H, K, L, dE. Instead of using a q-vector generator, they would prefer MDANSE just to load a list of HKL values which they already prepared elsewhere.

**Describe the solution you'd like**
A mock generator could be implemented, which takes as an input parameter the filename from which to load the vectors.

**Describe alternatives you've considered**
Going back to the way MDANSE used to work, we could then also save our own vector definitions as files and load them for analysis runs.

**Additional context**
The important thing is to save the q vectors together with the other input files, and not in a global definition directory.
The format of the q vector file remains to be specified.

Start by locating the existing q-vector generators and the analysis-run input handling. Define the q-vector file format, then determine how a mock generator can load a user-provided filename and keep the vectors with the other input files rather than in a global directory.

Written by the indexing model from the issue text.