rx_fm: read and demodulate raw I/Q from stdin or file

[rxseger]

To more easily test rtl_sdr (logs I/Q samples to a file, or stdout), would like rtl_fm to be able to read I/Q from a file (or '-' for stdin), as an alternative to reading from the device

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Note the input file format if implementing https://github.com/rxseger/rtl_tools/issues/8 'Support for non-CS8 stream formats (CS16, CF32)' + https://github.com/rxseger/rx_tools/pull/12 - possibly default to CU8, but add a flag or filename extension detection for CS16 and CF32

[rxseger]

Worth noting that rtl_fm has a raw I/Q output mode (-M iq or -M raw), which is documented as "raw mode outputs 2x16 bit IQ pairs" (which is CS16).

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In https://github.com/rxseger/rx_tools/pull/12, added to rx_sdr: -F CU8|CS8|CS16 output format selection, still defaulting to CU8 for compatibility with rtl_sdr, but CS16 has fewer conversions. Natively reading with SOAPY_SDR_CS16 to allow supporting >8-bit-only SDRs like bladeRF.

[Ulmo]

I installed rx_tools thinking this was already added, only to have to come back here and read in more detail to confirm it has not yet been added.

[spel-oe]

a feature to use all rx_tools with a stdin/fifo or file would be nice!