[mag] Make RM3100 module frequency variable

[matteobarbera]

This is a pull request addressing issue #2788. The module frequency is turned into a variable to allow the user to change it as they see fit to make use of the high polling frequency the RM3100 mag sensor is capable of.

With this change, the variable MODULE_RM3100_RATE can be defined in the airframe to change the default module frequency of 60 Hz.

<module name="mag" type="rm3100">
      <configure name="MODULE_RM3100_RATE" value="150"/>
</module>

I tested the PR by defining MODULE_RM3100_RATE and setting it to 60, 150, 300 to check that the frequency change works as intended.

[matteobarbera]

I wanted to also add a check for frequencies less than 1 in mag_rm3100.c, but for some reason this does not cause an error in compilation.

#if (MODULE_RM3100_RATE < 1)
#error "Module frequency cannot be less than 1 Hz"
#endif

[fvantienen]

The plot of 60Hz shows clearly that you read the same sensor measurement twice. I haven't looked at the code, butthis seems to me like there is an error in the implementation of the driver.

[gautierhattenberger]

Check this module to see how to make a configurable period with default value: https://github.com/paparazzi/paparazzi/blob/master/conf/modules/baro_ms5611_i2c.xml#L23

fixed