Use high resolution time for Timer::read().

[tkem]

Description

float Timer::read() delegates to Timer::read_us(), which returns an int, i.e. a signed 32-bit value capable of storing values up to 2147483647 us (the use of signed types in this interface has always bothered me, but that's outside the scope of this PR). Therefore, Timer::read() will return values up to ~2147.5f seconds, or 35.8 minutes, even though the float return type could represent a much higher range (3.402823e+38 seconds, at reduced precision, of course).

By contrast, int Timer::read_ms() returns read_high_resolution_us() / 1000, so the maximum (signed) value is 2147483647 ms, or more than 24 days (if I haven't made a mistake).

Pull request type

[x] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Breaking change

[kjbracey]

I don't like the float return here, but if we have it, this is clearly a better implementation.

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3177
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7975/

Triggering tests

/morph test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2775
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7975/

[cmonr]

/morph mbed2-build

[mbed-ci]

Test : SUCCESS

Build number : 2982
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7975/2982