Please expose gwmargin and ngws from MCC_LinkCheckAns

[BNNorman]

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I want to create a node which tells me the margin and number of gateways visible at the location of the node in order to produce a tool for identifying a good inner-city location for installing air-quality sensors. The number of gateways indicates the probable connection reliability of a site whilst the margin indicates the chances of successful signal decoding. I can get SNR and RSSI but gateway count is more important to me.

The MCMD_LinkCheckAns has the gwmargin and ngws return values commented out in lmic.c. I.e. the return values are ignored.

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Provide two public methods to return gwmargin and ngws. E.g. int getGwmargin() and int getNgws()

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I can modify my copy of the lmic.c and lmic.h source code to expose these values myself but, in platformio, when I choose different target devices I get multiple copies of this library in libdeps and would have to hand edit each one to expose gwmargin and ngws. (although I could just copy the changed files).

If this library is updated and silently updates the libdeps in platformio I will lose any local changes made.

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[BNNorman]

Closed in favour of #825