SIM7000 LTE Shield Versions Save

• Change initialization code to use AT+GMR to identify module type rather than ATI, because for SIM7070 ATI does not include module name
• Update LTE_Demo.ino and SIM7XXX_LTE_Demo.ino to reflect SIM7070 changes
• Test SIM7070 support: Connection with Hologram.io SIM card, sending/receiving SMS, network time, read battery level, general functions, HTTP GET requests (tested on T-Mobile LTE CAT-M1 network in the US)

Discovered issues with SIM7070G, most likely related to SIMCom firmware:

1. HTTP POST request according to example 1 in the SIM7070 HTTP(S) app note returns HTTP 400 code
2. HTTP POST request according to example 2 in the SIM7070 HTTP(S) app note does not accept JSON body and successfully posts, but with no content
3. GNSS commands do not run at all (return "ERROR")

• Fixed bugs (like incomplete readRTC function)
• Removed module variants for SIM7000 and above (for example, no more distinction in the library between SIM7000A and SIM7000G, or SIM7500A/E)
• Added more explicit commented hardware serial section in example code
• Added preliminary SIM7070 support and HTTP functions
• Added SSL for TCP (for MQTTS), thanks to @ikstvn
• General cleanup work

• Fixed a bunch of small things
• Added dedicated MQTT commands for SIM7000 and SIM7000_MQTT_Demo.ino example, including subscribing to topics and parsing incoming messages from subscribed topics
• Added printout of server response in postData() function after AT+HTTPREAD command

First official release!