ACAS-678: Use sketcher_clear API to clear structure in MaestroChemicalStructureController.

[iamanandkr]

In the ACAS Compound Registration clicking on "New Salt" button produces the following traceback,

Uncaught TypeError: this.maestro.clearSketcher is not a function
    at MaestroChemicalStructureController.clear (ACASFormChemicalStructure.js:131:27)
    at child.show (Salt.js:174:37)
    at child.showAddSaltPanel (SaltForm.js:229:27)
    at HTMLDivElement.dispatch (jquery-1.7.2.min.js:3:4816)
    at HTMLDivElement.i (jquery-1.7.2.min.js:3:709)

Description

Maestro sketcher has the clear_sketcher API to clear the sketcher. Here, we replace the usage with the correct API. We also clear the sketcher for showing, but I ran into the situation where maestro was still not defined, so for now I have added logic to only clear the sketcher if it's available.

Related Issue

ACSAS-678

How Has This Been Tested?

I was able to reproduce the error on qa-demo-23-2 and then verified after the changes the error is no longer seen. I was able to save a salt and then reopen the sketcher and the sketcher state was clear.

[bffrost]

LGTM!

[brianbolt]

LGTM