Add documentation for utilities

utils/README.md

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+# Utilities
+
+## Integrate
+The `Integrable` trait, Butcher Tableaus, and functions to integrate explicit partial differential equations. Embedded integration is additionally available, which allows for adaptive time-steps,although selection of timesteps is left to the users.
+
+## Constmatrix
+A constant-size matrix which supports matrix-multiplication.
+
+
+## Float
+The basic building-block for selection of the float type. `f32` can be selected for applications requiring lower precision, such as direct visualisation in graphics.
+
+## Fast-Float
+A wrapper-type around the unsafe float intrinsics