Add thread count variant to atlas (#9999)

* Add thread count variant

Atlas automatically configures itself and optimizes to run with a fixed number of threads. This can be overridden during compile by setting a flag and atlas will tune itself to use the specified number, rather than all system CPUs. 

Default value is the existing logic, autoconfigure.

* Fix Linting

* Another lint for longline

var/spack/repos/builtin/packages/atlas/package.py

@@ -46,6 +46,12 @@ class Atlas(Package):
         multi=False
     )
 
+    variant('tune_cpu', default=-1,
+        multi=False,
+        description="Number of threads to tune to,\
+                -1 for autodetect, 0 for no threading"
+    )
+
     provides('blas')
     provides('lapack')
 
@@ -77,6 +83,11 @@ def install(self, spec, prefix):
             '-b', '64'
         ])
 
+        # set number of cpu's to tune to
+        options.extend([
+            '-t', spec.variants['tune_cpu'].value
+        ])
+
         # set compilers:
         options.extend([
             '-C', 'ic', spack_cc,