Clean up terminology in interp1d module

The problem

Our terminology in the interp1d module is a bit confused. We should clean this up to help clarify the domain model and make our life easier in future.

Definition of "done"

• change x to time throughout the interp1d module
• re-name src/libfgen/interp1d to src/libfgen/1d_handling
• re-name src/libfgen/interp1d/base.f90 to src/libfgen/1d_handling/interp1d_base.f90
• check that the modules in src/libfgen/interp1d are 'symmetric' (i.e. if there is boundary stuff in next, it should also be in linear, even if it is only a comment/a dumb function which does nothing). This will help make it easier for developers to see what is going on unambiguously
• re-name Quadratic to QuadraticSpline, Linear to LinearSpline, Cubic to CubicSpline
• add docs that allow a user to more easily navigate and understand the domain that the 1d_handling covers
• make follow up issue for writing an overview documentation that covers the full domain (with plots etc. using e.g. sphinx plus python)

Additional context

!59 (comment 1788808535)