ENH: allow conversions between NIMADS <--> SLEUTH (#942)

* ENH: NIMADS↔Sleuth conversions, schema + pydantic validation, affine support, annotation splitting controls; add Sleuth→NIMADS + round‑trip content test; tidy tests and docstrings

* refactor code and tests

* add more conversion functions

* remove unused exception

* remove looking into raw

* fix leaky modification

Author: jdkent

nimare/exceptions.py

@@ -0,0 +1,9 @@
+"""Custom exceptions for NiMARE."""
+
+
+class NiMAREError(Exception):
+    """Base class for NiMARE exceptions."""
+
+
+class InvalidStudysetError(NiMAREError):
+    """Exception raised when a studyset is invalid or malformed."""