Add Flux from() and filter() methods

Signed-off-by: davidarendsen <davidarendsen@hey.com>

src/Expression/Base.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace Arendsen\FluxQueryBuilder\Expression;
+
+abstract class Base {
+
+	/**
+	 * @throws ExpressionNotImplementedException
+	 */
+	public function __toString()
+    {
+		throw new ExpressionNotImplementedException('__toString', get_class($this));
+	}
+
+}

src/Expression/ExpressionNotImplementedException.php

@@ -0,0 +1,7 @@
+<?php
+
+namespace Arendsen\FluxQueryBuilder\Expression;
+
+use Exception;
+
+class ExpressionNotImplementedException extends Exception {}

src/Expression/Filter/AndExpression.php

@@ -0,0 +1,30 @@
+<?php
+
+namespace Arendsen\FluxQueryBuilder\Expression\Filter;
+
+use Arendsen\FluxQueryBuilder\Expression\Base;
+
+class AndExpression extends Base {
+
+    /**
+     * @var string $key
+     */
+    private $key;
+
+    /**
+     * @var string $value
+     */
+    private $value;
+
+    public function __construct(string $key, string $value)
+    {
+        $this->key = $key;
+        $this->value = $value;
+    }
+
+    public function __toString()
+    {
+        return 'and r.' . $this->key . ' == "' . $this->value . '"';
+    }
+
+}

src/Expression/Filter/Measurement.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace Arendsen\FluxQueryBuilder\Expression\Filter;
+
+use Arendsen\FluxQueryBuilder\Expression\Base;
+
+class Measurement extends Base {
+
+    /**
+     * @var array $measurement
+     */
+    private $measurement;
+
+    public function __construct(string $measurement)
+    {
+        $this->measurement = $measurement;
+    }
+
+    public function __toString()
+    {
+        return 'r._measurement == "' . $this->measurement . '"';
+    }
+
+}

src/Expression/Filter/OrExpression.php

@@ -0,0 +1,30 @@
+<?php
+
+namespace Arendsen\FluxQueryBuilder\Expression\Filter;
+
+use Arendsen\FluxQueryBuilder\Expression\Base;
+
+class OrExpression extends Base {
+
+    /**
+     * @var string $key
+     */
+    private $key;
+
+    /**
+     * @var string $value
+     */
+    private $value;
+
+    public function __construct(string $key, string $value)
+    {
+        $this->key = $key;
+        $this->value = $value;
+    }
+
+    public function __toString()
+    {
+        return 'or r.' . $this->key . ' == "' . $this->value . '"';
+    }
+
+}