extend zonal reserves to include requirements that are functions of Pg

[rdzman]

As described in MOST issue #15, @lordleoo proposes to extend the zonal reserve implementation to include zonal requirements that are linear functions of Pg, in addition to the existing fixed requirements.

For zone k the original constraint (7.5) in the User's Manual ...
\sum_{i \in Z_k} r_i >= R_k
... is modified to ...
\sum_{i \in Z_k} r_i >= R_k + c_k * Pg
... where c_k is the k-th row of a new optional nrz x ng input matrix mpc.reserves.req_p.

Note that I'm suggesting to change the name of the new field from pweights (as initially proposed in MATPOWER/most#15) to req_p.

[rdzman]

In terms of documentation, this is going to require updating Section 7.6.1 in the User's Manual. The source for the manual can be found in docs/src/MATPOWER-manual/MATPOWER-manual.tex.

We should change "Fixed Zonal Reserves" to "Zonal Reserves" everywhere, add a new equation in that section to supersede equation (7.5), and add the new input to Table 7-5.

I think it would also be good to add a paragraph describing the common use cases.

A few other things that are needed:

• The columns of the new req_p field will need to be converted to and from internal indexing in the ext2int and int2ext callbacks in toggle_reserves (like we do with zones).
• The t_run_opf_w_res test should be updated to include cases that exercise the new feature.