Fix power pulse & reset issue

[giulcioffi]

This is intended to solve the issues (#47) related to the incorrect power-on/off and reset sequences of SARA-R410M module.
The modified library has been provided by @janakelarsson.
Then, the File System has been accordingly modified: the MODEM.begin(restart) function is always called, but the Sara module is restarted only when needed.

[aentinger]

Good morning @janakelarsson ☕ Would you be so kind and have a look as well as test this PR and let me know if it fixes your issues?

[janakelarsson]

As far as I can see, this fixes the issues in the library. It remains to fix the variant core.

[aentinger]

LGTM 👍 Thank you @giulcioffi and @janakelarsson 🚀 arduino/ArduinoCore-samd#549 has been merged so it should be fixed within the core.

[CCorrado]

@sandeepmistry @aentinger @giulcioffi ... is there a planned release with these critical changes? I had to use the tip of the master branch of MKRNB to avoid unrecoverable states due to the use of the reset pin in this library.

These commits are critical to use the MKRNB to prototype in the field. Without these changes, the board is an expensive paperweight...

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At the very least, a new release needs to be made available with the latest changes. In addition, it seems @janakelarsson has some other good improvements in PR that should be considered. Thoughts?