[wpimath] Add fmtlib formatter overloads for Eigen::Matrix and units (#3409)

This allows using Eigen matrices or units natively with fmt::format() or
fmt::print().

wpimath/src/main/native/include/frc/fmt/Eigen.h

@@ -0,0 +1,44 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#pragma once
+
+#include <fmt/format.h>
+
+#include "Eigen/Core"
+
+template <int Rows, int Cols, int... Args>
+struct fmt::formatter<Eigen::Matrix<double, Rows, Cols, Args...>> {
+  char presentation = 'f';
+
+  constexpr auto parse(fmt::format_parse_context& ctx) {
+    auto it = ctx.begin(), end = ctx.end();
+    if (it != end && (*it == 'f' || *it == 'e')) {
+      presentation = *it++;
+    }
+
+    if (it != end && *it != '}') {
+      throw fmt::format_error("invalid format");
+    }
+
+    return it;
+  }
+
+  template <typename FormatContext>
+  auto format(const Eigen::Matrix<double, Rows, Cols, Args...>& mat,
+              FormatContext& ctx) {
+    auto out = ctx.out();
+    for (int i = 0; i < Rows; ++i) {
+      for (int j = 0; j < Cols; ++j) {
+        out = fmt::format_to(out, "  {:f}", mat(i, j));
+      }
+
+      if (i < Rows - 1) {
+        out = fmt::format_to(out, "\n");
+      }
+    }
+
+    return out;
+  }
+};