Implements Session Parameters.

A SessionParam is a parameter that can modify MDX and SQL queries. The parameters are held in a SessionParamPool. There is one pool per HttpSession.

SessionParam may be created in several ways:

- by some application logic. For example at login-time there may be some parameters that restrict the data a user is allowed to see
- by clicking on a member in the JPivot table {@link com.tonbeller.jpivot.table.navi.ClickableMemberTag}
- by clicking on a hyperlink {@link com.tonbeller.jpivot.param.ParamLinkTag}

Parameters may be used in sql queries. Example for JSTL's sql tag:

    <jsp:useBean id="paramPool" class="com.tonbeller.jpivot.param.SessionParamPool" scope="session"/>

    <sql:query var="result" dataSource="${myDataSource}">
      SELECT ...
      FROM ...
      WHERE CUSTOMERID = ?
      <sql:param value="${paramPool.CUSTOMERID.sqlValue}"/>
    </sql:query>