Lighten/improve the infrastructure

[dreamer2368]

Several milestones to lighten/improve the infrastructure of the code:

Remove TrainMode

TrainMode::INDIVIDUAL refers to training POD basis per individual subdomain. Even if some subdomains are of the same component, they collect different snapshots and train different POD basis.

In actual production runs, TrainMode::INDIVIDUAL is not really used any more. Furthermore, the same feature can be executed from TrainMode::UNIVERSAL, just using the same mesh for two different components (at the different location). For SubmeshTopologyHandler, this means we're having the same number of components as the number of subdomains, as opposed to one component representing all subdomains.

Having two (unnecessary) different modes and supporting them throughout the workflow is complicating the overall code. The overall structure can be lightened by removing this feature.

Datatype for BasisTag

Unlike single-component ROM, ROM basis for multi-component system has multiple tags: by its component, by variable. The aforementioned TrainMode also changes the component name, further complicating the code.

Considering a future development for interior/interface basis separation, we would want to make a struct for basis tag, containing its component, variable, etc.

[dreamer2368]

Addressed by #47 and #48 .