Remove NaNs and process hanging

[jj1bdx]

This pull request includes the fixes for the following bugs:

• Causing a hung process during the startup period before valid audio signals are coming out
• Causing displaying -nan in the output level meter in broadcast FM and NBFM
  • The NaN is presumably generated by volk_32fc_s32f_atan2_32f() in PhaseDiscriminator::process()
  • This bug also causes output disruption when the multipath filter is activated for broadcast FM

This pull request includes the following enhancements:

• Streamlining processing flow in the main for loop of main()
• Removing the initial waiting period for startup; the output is now activated from the block number 1
• Adding Utility::remove_nans(), a function to check and substitute NaNs and infinity values in IQSamplesDecodedVector