Add options -x and -e in 'taurus tpg trend'

[mrosanes]

Manual tests of 'taurus' and 'taurus tpg' (in taurus/doc/how_to_release.md) includes the test of:

`taurus tpg trend -xe "eval:Q(rand(),'mm')" sys/tg_test/1/ampli`

However options -x and -e are not still available in 'taurus tpg trend'. Include options -x and -e in 'taurus tpg trend'.

[cpascual]

That was a typo in the "how_to_release" document (now fixed).

Where it said "-xe" it should say "-xn". The -x option does accept the n value, so this can be closed