Issue/19/informer module

Change & Solution Description

This addresses #19 by moving the informers to their own module so new developers can see all the types, distinguished by their inputs rather than their outputs and thus usable by an Estimator, Summarizer, or Classifier. I added a line to import the ones that used to be in estimator.py and summarizer.py in hopes that the path changes wouldn't break existing estimators/summarizers/classifiers.

Code Quality

[x] My code builds (or compiles) cleanly without any errors or warnings
[ ] My code has complete unit test coverage, or I have justified any instances of #pragma: no cover
[x] My code contains relevant comments and necessary documentation for future maintainers
[x] My docstrings use the NumPy docstring format
[x] If I have made a breaking change, I include backwards compatibility and deprecation warnings (if possible)

Codecov Report

Patch coverage: 82.35% and project coverage change: -1.68% :warning:

Comparison is base (3248a0d) 97.53% compared to head (25e9b87) 95.86%.

:exclamation: Current head 25e9b87 differs from pull request most recent head ba379a7. Consider uploading reports for the commit ba379a7 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #34 +/- ## ========================================== - Coverage 97.53% 95.86% -1.68% ========================================== Files 28 29 +1 Lines 1585 1643 +58 ========================================== + Hits 1546 1575 +29 - Misses 39 68 +29 ``` | [Files Changed](https://app.codecov.io/gh/LSSTDESC/rail_base/pull/34?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LSSTDESC) | Coverage Δ | | |---|---|---| | [src/rail/estimation/informer.py](https://app.codecov.io/gh/LSSTDESC/rail_base/pull/34?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LSSTDESC#diff-c3JjL3JhaWwvZXN0aW1hdGlvbi9pbmZvcm1lci5weQ==) | `80.00% <80.00%> (ø)` | | | [src/rail/estimation/estimator.py](https://app.codecov.io/gh/LSSTDESC/rail_base/pull/34?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LSSTDESC#diff-c3JjL3JhaWwvZXN0aW1hdGlvbi9lc3RpbWF0b3IucHk=) | `100.00% <100.00%> (ø)` | | | [src/rail/estimation/summarizer.py](https://app.codecov.io/gh/LSSTDESC/rail_base/pull/34?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LSSTDESC#diff-c3JjL3JhaWwvZXN0aW1hdGlvbi9zdW1tYXJpemVyLnB5) | `58.92% <100.00%> (-41.08%)` | :arrow_down: |

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LSSTDESC / rail_base