add missing extrenal database links to orphan metabolites

[eladnoor]

It seems that quite a few annotations can be added, even to metabolites that are in the latest E. coli model (iML1515). This is important for applications such as eQuilibrator and kinetic modeling, where compound annotations are used to automatically retrieve external information (thermodynamics, kinetics, concentrations, etc.).

For example:
nadphx__S has not annotations, but exists in MetaNetX as MNXM2897. Likewise for nadphx__R --> MNXM6524. This is a partial list of missing annotations I found just in the iML1515 model:

BIGG2EXTERNAL = {
    "ttdca_c": "CHEBI:30807",
    "nadphx__S_c": "CHEBI:64076",
    "nadphx__R_c": "CHEBI:64085",
    "nadhx__S_c": "CHEBI:44236",
    "nadhx__R_c": "CHEBI:134125",
    "cdpdhdecg_c": "CHEBI:77190",
    "cdpdddecg_c": "metanetx.chemical:MNXM4507",
    "hxcoa_c": "CHEBI:62620",
    "dkdgln_c": "ChEBI:15623",
    "dt5hsu_c": "CHEBI:17117",
    "dxylnt_c": "CHEBI:48093",
    "2ddara_c": "CHEBI:16699",
    "urdgci_c": "CHEBI:16282",
    "metglcur_c": "CHEBI:183134",
    "psuri_c": "CHEBI:17802",
    "psuri_e": "CHEBI:17802",
    "psuri_p": "CHEBI:17802",
    "f_c": "CHEBI:17051",
    "f_p": "CHEBI:17051",
    "sq_c": "CHEBI:77132",
    "sf_c": "CHEBI:77261",
    "sfp_c": "CHEBI:77263",
    "sla_c": "CHEBI:77265",
    "sq_p": "CHEBI:77132",
    "dhptdp_c": "KEGG:C20959",
    "dhptdd_c": "KEGG:C20960",
    "dtgcl_c": "CHEBI:43136",
    "mercpeth_c": "CHEBI:41218",
    "dhcholn_c": "CHEBI:16390",
    "4abzglu_c": "CHEBI:60903",
    "5phdt_c": "CHEBI:140750",
    "5phua_c": "CHEBI:140758",
    "teo2_c": "CHEBI:30477",
    "mteo2_c": "CHEBI:71624",
    "34dphacoa_c": "CHEBI:189558",
    "mchtbs6p_c": "CHEBI:71317",
    "cur_c": "CHEBI:3962",
    "dhcur_c": "CHEBI:67262",
    "lkdr_c": "CHEBI:58371",
    "acmet_c": "CHEBI:132958",
    "iprimv_c": "CHEBI:146431",
    "erthrs_c": "CHEBI:27904",
    "puacgam_c": "CHEBI:67063",
    "dhps_c": "KEGG:C22383",
    "dhps_e": "KEGG:C22383",
    "cs1_e": "CHEBI:49547",
    "r2hglut_c": "CHEBI:15801",
    "prpmn_c": "KEGG:C20423",
    "prcp_c": "KEGG:C20440",
    "gmplys_c": "CHEBI:64856",
    "nalme_c": "CHEBI:64854",
    "mepn_c": "CHEBI:45129",
    "rpntp_c": "CHEBI:68685",
    "udcpgl_c": "KEGG:C19772",
    "4abzglu_e": "CHEBI:60903",
    "metglcur_e": "CHEBI:183134",
    "metglcur_p": "CHEBI:183134",
    "thcur_c": "CHEBI:67263",
    "rhmn_c": "CHEBI:58118",
    "3hbzcoa_c": "CHEBI:15484",
    "3hbz_c": "CHEBI:16193",
    "ampnt_c": "CHEBI:133674",
    "acampnt_c": "CHEBI:134093",
    "dkdglcn_c": "CHEBI:15623",
}