added useful PerpleX messages

[bobmyhill]

PerpleX 7.x has slightly changed the functionality and expected user input of werami. This PR alerts the user to the following things they must now do to prepare their directory before running the burnman.perplex.create_perplex_table():

• run vertex
• set sample_on_grid to F in the perplex option file if they want to use the default P-T range

The code itself is reformatted slightly to allow easier processing of the stdout from werami.

This PR is a first step toward addressing #524.

The code works with Perple_X release 7.0.9.

[vivianotieno]

Dear bobmyhill

Thank you for the feedback. I'll try out the recommended solution and then let you know how it goes.

[vivianotieno]

Dear Bobmyhill

I'm sorry for not getting back to you sooner. The new version works. I didn't have to change the sample grid mode.