ENH: add support for specifying dtypes

[mj-will]

Add dtype option to the main Aspire class.

This ensures the same dtype is used throughout the code. In future, one could consider allowing different dtypes for e.g. the likelihood and flow. The current version should be enough to robustly sample geocent_time in GW problems.

Changes

• Utils for resolving and converting dtypes
• dtype keyword arguments in most classes
• Refactor bounded transforms to check precision is sufficient