Update main PEcAn documentation to reflect custom pecan.xml requirements for MAAT model

[serbinsh]

Updates to the PEcAn documentation with respect to MAAT are needed for the customizations needed in pecan.xml for running MAAT. For example: https://github.com/PecanProject/pecan/blob/develop/models/maat/vignettes/running_maat_in_pecan.Rmd

Specifically adding additional tag options under

      <config>
        <mod_obj>leaf</mod_obj>
        <leaf>
          <fnames>
            <vcmax>'f_vcmax_constant'</vcmax>
            <jmax>'f_jmax_constant'</jmax>
            <rd>'f_rd_constant'</rd>
            <rs>'f_rs_ball1987'</rs>
            <etrans>'f_j_farquharwong1984'</etrans>
            <tcor_asc>
              <vcmax>'f_tcor_asc_Arrhenius'</vcmax>
              <jmax>'f_tcor_asc_Arrhenius'</jmax>
              <rd>'f_tcor_asc_Q10'</rd>
              <tpu>'f_tcor_asc_Arrhenius'</tpu>
            </tcor_asc>
            <tcor_des>
              <vcmax>'f_tcor_des_modArrhenius'</vcmax>
              <jmax>'f_tcor_des_modArrhenius'</jmax>
              <rd>'f_tcor_des_cox2001'</rd>
           </tcor_des>
          </fnames>
        </leaf>
    </config>

Can or need to point to main MAAT documentation that explains the various parameter names, fnames (function) options, etc that can be enabled in MAAT.

Also need to add some basic defaults in write.configs() and warning messages about using basic defaults.

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[robkooper]

I think we are ok just mentioning something about how anything in the config section of a model can be used by the model write config, and leave the documentation of these parameters up to the model itself.

  <model>
    <config>
      ....
    </config>
  </model>